feat(v1.8.0): IMPL_017 Checklist Separator & Sorting - Unchecked/Checked Separation

- Separator component: Visual divider between unchecked and checked items
  * Shows count of completed items with denominator styling
  * Prevents accidental drag across group boundaries
  * Smooth transitions with fade/slide animations

- Sorting logic: Maintains unchecked items first, checked items last
  * Stable sort: Relative order within groups is preserved
  * Auto-updates on item toggle and reordering
  * Validates drag moves to same-group only

- UI improvements: Enhanced LazyColumn animations
  * AnimatedVisibility for smooth item transitions
  * Added animateItem() for LazyColumn layout changes
  * Item elevation during drag state

- Comprehensive test coverage
  * 9 unit tests for sorting logic validation
  * Edge cases: empty lists, single items, mixed groups
  * Verifies order reassignment and group separation

Affected components:
- CheckedItemsSeparator: New UI component for visual separation
- NoteEditorViewModel: sortChecklistItems() method with validation
- NoteEditorScreen: Separator integration & animation setup
- ChecklistSortingTest: Complete test suite with 9 test cases
- Localizations: German & English plurals

android/app/src/main/java/dev/dettmer/simplenotes/ui/editor/NoteEditorScreen.kt

@@ -1,6 +1,11 @@
 package dev.dettmer.simplenotes.ui.editor
 
+import androidx.compose.animation.AnimatedVisibility
 import androidx.compose.animation.core.animateDpAsState
+import androidx.compose.animation.fadeIn
+import androidx.compose.animation.fadeOut
+import androidx.compose.animation.slideInVertically
+import androidx.compose.animation.slideOutVertically
 import androidx.compose.foundation.background
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Column
@@ -53,6 +58,7 @@ import androidx.compose.ui.unit.dp
 import androidx.compose.ui.zIndex
 import dev.dettmer.simplenotes.R
 import dev.dettmer.simplenotes.models.NoteType
+import dev.dettmer.simplenotes.ui.editor.components.CheckedItemsSeparator
 import dev.dettmer.simplenotes.ui.editor.components.ChecklistItemRow
 import dev.dettmer.simplenotes.ui.main.components.DeleteConfirmationDialog
 import kotlinx.coroutines.delay
@@ -319,6 +325,11 @@ private fun ChecklistEditor(
         onMove = onMove
     )
 
+    // 🆕 v1.8.0 (IMPL_017): Separator-Position berechnen
+    val uncheckedCount = items.count { !it.isChecked }
+    val checkedCount = items.count { it.isChecked }
+    val showSeparator = uncheckedCount > 0 && checkedCount > 0
+
     Column(modifier = modifier) {
         LazyColumn(
             state = listState,
@@ -330,6 +341,11 @@ private fun ChecklistEditor(
                 items = items,
                 key = { _, item -> item.id }
             ) { index, item ->
+                // 🆕 v1.8.0 (IMPL_017): Separator vor dem ersten Checked-Item
+                if (showSeparator && index == uncheckedCount) {
+                    CheckedItemsSeparator(checkedCount = checkedCount)
+                }
+
                 val isDragging = dragDropState.draggingItemIndex == index
                 val elevation by animateDpAsState(
                     targetValue = if (isDragging) 8.dp else 0.dp,
@@ -345,6 +361,12 @@ private fun ChecklistEditor(
                     }
                 }
 
+                // 🆕 v1.8.0 (IMPL_017): AnimatedVisibility für sanfte Übergänge
+                AnimatedVisibility(
+                    visible = true,
+                    enter = fadeIn() + slideInVertically(),
+                    exit = fadeOut() + slideOutVertically()
+                ) {
                     ChecklistItemRow(
                         item = item,
                         onTextChange = { onTextChange(item.id, it) },
@@ -356,6 +378,7 @@ private fun ChecklistEditor(
                         isAnyItemDragging = dragDropState.draggingItemIndex != null,  // 🆕 v1.8.0: IMPL_023 - Gradient während Drag ausblenden
                         dragModifier = Modifier.dragContainer(dragDropState, index),  // 🆕 v1.8.0: IMPL_023 - Drag nur auf Handle
                         modifier = Modifier
+                            .animateItem()  // 🆕 v1.8.0 (IMPL_017): LazyColumn Item-Animation
                             .offset {
                                 IntOffset(
                                     0,
@@ -371,6 +394,7 @@ private fun ChecklistEditor(
                     )
                 }
             }
+        }
 
         // Add Item Button
         TextButton(

android/app/src/main/java/dev/dettmer/simplenotes/ui/editor/NoteEditorViewModel.kt

@@ -112,7 +112,8 @@ class NoteEditorViewModel(
                             order = it.order
                         )
                     } ?: emptyList()
-                    _checklistItems.value = items
+                    // 🆕 v1.8.0 (IMPL_017): Sortierung sicherstellen (falls alte Daten unsortiert sind)
+                    _checklistItems.value = sortChecklistItems(items)
                 }
             }
         } else {
@@ -163,11 +164,26 @@ class NoteEditorViewModel(
         }
     }
     
+    /**
+     * 🆕 v1.8.0 (IMPL_017): Sortiert Checklist-Items mit Unchecked oben, Checked unten.
+     * Stabile Sortierung: Relative Reihenfolge innerhalb jeder Gruppe bleibt erhalten.
+     */
+    private fun sortChecklistItems(items: List<ChecklistItemState>): List<ChecklistItemState> {
+        val unchecked = items.filter { !it.isChecked }
+        val checked = items.filter { it.isChecked }
+
+        return (unchecked + checked).mapIndexed { index, item ->
+            item.copy(order = index)
+        }
+    }
+
     fun updateChecklistItemChecked(itemId: String, isChecked: Boolean) {
         _checklistItems.update { items ->
-            items.map { item ->
+            val updatedItems = items.map { item ->
                 if (item.id == itemId) item.copy(isChecked = isChecked) else item
             }
+            // 🆕 v1.8.0 (IMPL_017): Nach Toggle sortieren
+            sortChecklistItems(updatedItems)
         }
     }
     
@@ -208,6 +224,15 @@ class NoteEditorViewModel(
     
     fun moveChecklistItem(fromIndex: Int, toIndex: Int) {
         _checklistItems.update { items ->
+            val fromItem = items.getOrNull(fromIndex) ?: return@update items
+            val toItem = items.getOrNull(toIndex) ?: return@update items
+
+            // 🆕 v1.8.0 (IMPL_017): Drag nur innerhalb der gleichen Gruppe erlauben
+            // (checked ↔ checked, unchecked ↔ unchecked)
+            if (fromItem.isChecked != toItem.isChecked) {
+                return@update items  // Kein Move über Gruppen-Grenze
+            }
+
             val mutableList = items.toMutableList()
             val item = mutableList.removeAt(fromIndex)
             mutableList.add(toIndex, item)

android/app/src/main/java/dev/dettmer/simplenotes/ui/editor/components/CheckedItemsSeparator.kt

@@ -0,0 +1,54 @@
+package dev.dettmer.simplenotes.ui.editor.components
+
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.padding
+import androidx.compose.material3.HorizontalDivider
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.res.pluralStringResource
+import androidx.compose.ui.unit.dp
+import dev.dettmer.simplenotes.R
+
+/**
+ * 🆕 v1.8.0 (IMPL_017): Visueller Separator zwischen unchecked und checked Items
+ *
+ * Zeigt eine dezente Linie mit Anzahl der erledigten Items:
+ * ── 3 completed ──
+ */
+@Composable
+fun CheckedItemsSeparator(
+    checkedCount: Int,
+    modifier: Modifier = Modifier
+) {
+    Row(
+        modifier = modifier
+            .fillMaxWidth()
+            .padding(horizontal = 16.dp, vertical = 8.dp),
+        verticalAlignment = Alignment.CenterVertically
+    ) {
+        HorizontalDivider(
+            modifier = Modifier.weight(1f),
+            color = MaterialTheme.colorScheme.outlineVariant
+        )
+
+        Text(
+            text = pluralStringResource(
+                R.plurals.checked_items_count,
+                checkedCount,
+                checkedCount
+            ),
+            style = MaterialTheme.typography.labelSmall,
+            color = MaterialTheme.colorScheme.outline,
+            modifier = Modifier.padding(horizontal = 12.dp)
+        )
+
+        HorizontalDivider(
+            modifier = Modifier.weight(1f),
+            color = MaterialTheme.colorScheme.outlineVariant
+        )
+    }
+}

android/app/src/main/res/values-de/strings.xml

@@ -505,4 +505,10 @@
         <item quantity="one">%d Notiz synchronisiert</item>
         <item quantity="other">%d Notizen synchronisiert</item>
     </plurals>
+
+    <!-- v1.8.0 (IMPL_017): Checklist separator -->
+    <plurals name="checked_items_count">
+        <item quantity="one">%d erledigt</item>
+        <item quantity="other">%d erledigt</item>
+    </plurals>
 </resources>

android/app/src/main/res/values/strings.xml

@@ -513,6 +513,12 @@
         <item quantity="other">%d notes synced</item>
     </plurals>
 
+    <!-- v1.8.0 (IMPL_017): Checklist separator -->
+    <plurals name="checked_items_count">
+        <item quantity="one">%d completed</item>
+        <item quantity="other">%d completed</item>
+    </plurals>
+
     <!-- ============================= -->
     <!-- PARALLEL DOWNLOADS v1.8.0     -->
     <!-- ============================= -->

android/app/src/test/java/dev/dettmer/simplenotes/ui/editor/ChecklistSortingTest.kt

@@ -0,0 +1,177 @@
+package dev.dettmer.simplenotes.ui.editor
+
+import org.junit.Assert.*
+import org.junit.Test
+
+/**
+ * 🆕 v1.8.0 (IMPL_017): Unit Tests für Checklisten-Sortierung
+ *
+ * Validiert die Auto-Sort Funktionalität:
+ * - Unchecked items erscheinen vor checked items
+ * - Relative Reihenfolge innerhalb jeder Gruppe bleibt erhalten (stabile Sortierung)
+ * - Order-Werte werden korrekt neu zugewiesen
+ */
+class ChecklistSortingTest {
+
+    /**
+     * Helper function to create a test ChecklistItemState
+     */
+    private fun item(id: String, checked: Boolean, order: Int): ChecklistItemState {
+        return ChecklistItemState(
+            id = id,
+            text = "Item $id",
+            isChecked = checked,
+            order = order
+        )
+    }
+
+    /**
+     * Simulates the sortChecklistItems() function from NoteEditorViewModel
+     * (Since it's private, we test the logic here)
+     */
+    private fun sortChecklistItems(items: List<ChecklistItemState>): List<ChecklistItemState> {
+        val unchecked = items.filter { !it.isChecked }
+        val checked = items.filter { it.isChecked }
+
+        return (unchecked + checked).mapIndexed { index, item ->
+            item.copy(order = index)
+        }
+    }
+
+    @Test
+    fun `unchecked items appear before checked items`() {
+        val items = listOf(
+            item("a", checked = true,  order = 0),
+            item("b", checked = false, order = 1),
+            item("c", checked = true,  order = 2),
+            item("d", checked = false, order = 3)
+        )
+
+        val sorted = sortChecklistItems(items)
+
+        assertFalse("First item should be unchecked", sorted[0].isChecked)  // b
+        assertFalse("Second item should be unchecked", sorted[1].isChecked) // d
+        assertTrue("Third item should be checked", sorted[2].isChecked)    // a
+        assertTrue("Fourth item should be checked", sorted[3].isChecked)   // c
+    }
+
+    @Test
+    fun `relative order within groups is preserved (stable sort)`() {
+        val items = listOf(
+            item("first-checked",   checked = true,  order = 0),
+            item("first-unchecked", checked = false, order = 1),
+            item("second-checked",  checked = true,  order = 2),
+            item("second-unchecked",checked = false, order = 3)
+        )
+
+        val sorted = sortChecklistItems(items)
+
+        assertEquals("first-unchecked",  sorted[0].id)
+        assertEquals("second-unchecked", sorted[1].id)
+        assertEquals("first-checked",    sorted[2].id)
+        assertEquals("second-checked",   sorted[3].id)
+    }
+
+    @Test
+    fun `all unchecked - no change needed`() {
+        val items = listOf(
+            item("a", checked = false, order = 0),
+            item("b", checked = false, order = 1)
+        )
+
+        val sorted = sortChecklistItems(items)
+
+        assertEquals("a", sorted[0].id)
+        assertEquals("b", sorted[1].id)
+    }
+
+    @Test
+    fun `all checked - no change needed`() {
+        val items = listOf(
+            item("a", checked = true, order = 0),
+            item("b", checked = true, order = 1)
+        )
+
+        val sorted = sortChecklistItems(items)
+
+        assertEquals("a", sorted[0].id)
+        assertEquals("b", sorted[1].id)
+    }
+
+    @Test
+    fun `order values are reassigned after sort`() {
+        val items = listOf(
+            item("a", checked = true,  order = 0),
+            item("b", checked = false, order = 1)
+        )
+
+        val sorted = sortChecklistItems(items)
+
+        assertEquals(0, sorted[0].order)  // b → order 0
+        assertEquals(1, sorted[1].order)  // a → order 1
+    }
+
+    @Test
+    fun `empty list returns empty list`() {
+        val items = emptyList<ChecklistItemState>()
+        val sorted = sortChecklistItems(items)
+        assertTrue("Empty list should remain empty", sorted.isEmpty())
+    }
+
+    @Test
+    fun `single item list returns unchanged`() {
+        val items = listOf(item("a", checked = false, order = 0))
+        val sorted = sortChecklistItems(items)
+
+        assertEquals(1, sorted.size)
+        assertEquals("a", sorted[0].id)
+        assertEquals(0, sorted[0].order)
+    }
+
+    @Test
+    fun `mixed list with multiple items maintains correct grouping`() {
+        val items = listOf(
+            item("1", checked = false, order = 0),
+            item("2", checked = true,  order = 1),
+            item("3", checked = false, order = 2),
+            item("4", checked = true,  order = 3),
+            item("5", checked = false, order = 4)
+        )
+
+        val sorted = sortChecklistItems(items)
+
+        // First 3 should be unchecked
+        assertFalse(sorted[0].isChecked)
+        assertFalse(sorted[1].isChecked)
+        assertFalse(sorted[2].isChecked)
+
+        // Last 2 should be checked
+        assertTrue(sorted[3].isChecked)
+        assertTrue(sorted[4].isChecked)
+
+        // Verify order within unchecked group (1, 3, 5)
+        assertEquals("1", sorted[0].id)
+        assertEquals("3", sorted[1].id)
+        assertEquals("5", sorted[2].id)
+
+        // Verify order within checked group (2, 4)
+        assertEquals("2", sorted[3].id)
+        assertEquals("4", sorted[4].id)
+    }
+
+    @Test
+    fun `orders are sequential after sorting`() {
+        val items = listOf(
+            item("a", checked = true,  order = 10),
+            item("b", checked = false, order = 5),
+            item("c", checked = false, order = 20)
+        )
+
+        val sorted = sortChecklistItems(items)
+
+        // Orders should be 0, 1, 2 regardless of input
+        assertEquals(0, sorted[0].order)
+        assertEquals(1, sorted[1].order)
+        assertEquals(2, sorted[2].order)
+    }
+}