v1.3.1 - Multi-Device Sync Fix + Performance + Restore Bug Fix

🔧 Fixed:
- Multi-device JSON sync now works (thanks Thomas!)
- Restore from Server skipped files (timestamp bug)
- No duplicate downloads
- First MD sync after export now fast

⚡ Performance:
- JSON sync: 12-14s → 2-3s
- Hybrid timestamp + E-Tag optimization
- Matches Markdown sync speed

✨ New:
- Sync status UI in MainActivity
- Content-based MD import
- Debug logging improvements
- SyncStateManager for sync coordination

🔧 Technical:
- Clear lastSyncTimestamp on restore
- Clear E-Tag caches on restore
- E-Tag refresh after upload
- Fixed timestamp update after MD export

android/app/src/main/res/layout/activity_main.xml

@@ -22,6 +22,39 @@
             app:title="@string/app_name"
             app:titleTextAppearance="@style/TextAppearance.Material3.TitleLarge" />
 
+        <!-- 🔄 v1.3.1: Sync Status Banner -->
+        <LinearLayout
+            android:id="@+id/syncStatusBanner"
+            android:layout_width="match_parent"
+            android:layout_height="wrap_content"
+            android:orientation="horizontal"
+            android:gravity="center_vertical"
+            android:paddingHorizontal="16dp"
+            android:paddingVertical="8dp"
+            android:background="?attr/colorPrimaryContainer"
+            android:visibility="gone">
+
+            <com.google.android.material.progressindicator.CircularProgressIndicator
+                android:id="@+id/syncProgressIndicator"
+                android:layout_width="24dp"
+                android:layout_height="24dp"
+                android:indeterminate="true"
+                app:indicatorSize="24dp"
+                app:trackThickness="3dp"
+                app:indicatorColor="?attr/colorOnPrimaryContainer" />
+
+            <TextView
+                android:id="@+id/syncStatusText"
+                android:layout_width="0dp"
+                android:layout_height="wrap_content"
+                android:layout_weight="1"
+                android:layout_marginStart="12dp"
+                android:text="@string/sync_status_syncing"
+                android:textAppearance="@style/TextAppearance.Material3.BodyMedium"
+                android:textColor="?attr/colorOnPrimaryContainer" />
+
+        </LinearLayout>
+
     </com.google.android.material.appbar.AppBarLayout>
 
     <!-- SwipeRefreshLayout für Pull-to-Refresh (v1.1.2) -->