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
@@ -318,7 +324,12 @@ private fun ChecklistEditor(
         scope = scope,
         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,
@@ -330,48 +341,61 @@ 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,
                     label = "elevation"
                 )
-                
+
                 val shouldFocus = item.id == focusNewItemId
-                
+
                 // v1.5.0: Clear focus request after handling
                 LaunchedEffect(shouldFocus) {
                     if (shouldFocus) {
                         onFocusHandled()
                     }
                 }
-                
-                ChecklistItemRow(
-                    item = item,
-                    onTextChange = { onTextChange(item.id, it) },
-                    onCheckedChange = { onCheckedChange(item.id, it) },
-                    onDelete = { onDelete(item.id) },
-                    onAddNewItem = { onAddNewItemAfter(item.id) },
-                    requestFocus = shouldFocus,
-                    isDragging = isDragging,  // 🆕 v1.8.0: IMPL_023 - Drag state übergeben
-                    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
-                        .offset {
-                            IntOffset(
-                                0,
-                                if (isDragging) dragDropState.draggingItemOffset.roundToInt() else 0
+
+                // 🆕 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) },
+                        onCheckedChange = { onCheckedChange(item.id, it) },
+                        onDelete = { onDelete(item.id) },
+                        onAddNewItem = { onAddNewItemAfter(item.id) },
+                        requestFocus = shouldFocus,
+                        isDragging = isDragging,  // 🆕 v1.8.0: IMPL_023 - Drag state übergeben
+                        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,
+                                    if (isDragging) dragDropState.draggingItemOffset.roundToInt() else 0
+                                )
+                            }
+                            .zIndex(if (isDragging) 10f else 0f)  // 🆕 v1.8.0: IMPL_023 - Gedraggtes Item liegt über anderen
+                            .shadow(elevation, shape = RoundedCornerShape(8.dp))
+                            .background(
+                                color = MaterialTheme.colorScheme.surface,
+                                shape = RoundedCornerShape(8.dp)
                             )
-                        }
-                        .zIndex(if (isDragging) 10f else 0f)  // 🆕 v1.8.0: IMPL_023 - Gedraggtes Item liegt über anderen
-                        .shadow(elevation, shape = RoundedCornerShape(8.dp))
-                        .background(
-                            color = MaterialTheme.colorScheme.surface,
-                            shape = RoundedCornerShape(8.dp)
-                        )
-                )
+                    )
+                }
             }
         }
-        
+
         // Add Item Button
         TextButton(
             onClick = onAddItemAtEnd,

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

@@ -104,7 +104,7 @@ class NoteEditorViewModel(
                 }
                 
                 if (note.noteType == NoteType.CHECKLIST) {
-                    val items = note.checklistItems?.sortedBy { it.order }?.map { 
+                    val items = note.checklistItems?.sortedBy { it.order }?.map {
                         ChecklistItemState(
                             id = it.id,
                             text = it.text,
@@ -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     -->
     <!-- ============================= -->