[skip ci] feat: new app icon with monochrome support & updated descriptions

🎨 New App Icon:
- Fresh adaptive icon design with warm background (#f9e9c8)
- Monochrome icon support for Android 13+ themed icons
- PNG format replacing WebP for better compatibility
- All densities: mdpi, hdpi, xhdpi, xxhdpi, xxxhdpi

📝 Updated Descriptions (EN/DE):
- Added Multi-Device Sync feature
- Added Markdown export for Obsidian/desktop editors
- Added deletion tracking (zombie notes prevention)
- Added E-Tag caching (20x faster checks)
- Added optimized performance (~2-3s sync time)
- Added live sync status indicator
- Added Server-Restore modes (Merge/Replace/Overwrite)

📦 F-Droid Metadata:
- Updated build entries for v1.2.1, v1.2.2, v1.3.0, v1.3.1
- CurrentVersion now 1.3.1 (versionCode 9)
- Prepared for F-Droid merge request submission

android/app/src/main/res/mipmap-anydpi-v26/ic_launcher.xml

@@ -1,5 +1,5 @@
-<?xml version="1.0" encoding="utf-8"?>
 <adaptive-icon xmlns:android="http://schemas.android.com/apk/res/android">
-    <background android:drawable="@mipmap/ic_launcher_background"/>
-    <foreground android:drawable="@mipmap/ic_launcher_foreground"/>
+  <background android:drawable="@color/ic_launcher_background"/>
+  <foreground android:drawable="@mipmap/ic_launcher_foreground"/>
+  <monochrome android:drawable="@mipmap/ic_launcher_monochrome"/>
 </adaptive-icon>

android/app/src/main/res/mipmap-anydpi-v26/ic_launcher_round.xml

@@ -1,5 +1,5 @@
-<?xml version="1.0" encoding="utf-8"?>
 <adaptive-icon xmlns:android="http://schemas.android.com/apk/res/android">
-    <background android:drawable="@mipmap/ic_launcher_background"/>
-    <foreground android:drawable="@mipmap/ic_launcher_foreground"/>
+  <background android:drawable="@color/ic_launcher_background"/>
+  <foreground android:drawable="@mipmap/ic_launcher_foreground"/>
+  <monochrome android:drawable="@mipmap/ic_launcher_monochrome"/>
 </adaptive-icon>

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

@@ -4,6 +4,9 @@
     <color name="black">#FF000000</color>
     <color name="white">#FFFFFFFF</color>
     
+    <!-- Adaptive Icon Background -->
+    <color name="ic_launcher_background">#f9e9c8</color>
+    
     <!-- Material 3 Light Theme Colors -->
     <color name="md_theme_light_primary">#6750A4</color>
     <color name="md_theme_light_onPrimary">#FFFFFF</color>