v1.0.25: multi-select files, unified notification, dark theme, icon redesign

- FilesScreen: long-press enters selection mode, bulk share/delete toolbar, BackHandler
- FilesViewModel: ShareMultiple action, isSelectionMode/selectedCount state, download-to-cache for open/share
- FileWatchService: recursive FileObserver per subdirectory; unified notification updated with sync result via WorkManager flow observation
- SyncWorker: silent flag suppresses notifications when triggered by watcher; emits KEY_RESULT_SUMMARY output data
- Passbolt-inspired dark theme (Red700/Red500 primary, near-black surface)
- App icon: circular AutoSync-style sync arrows (cyan gradient, deep navy background)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/src/main/res/drawable/ic_launcher_background.xml

@@ -3,9 +3,8 @@
     <gradient
         android:type="radial"
         android:gradientRadius="80%"
-        android:centerX="0.35"
-        android:centerY="0.3"
-        android:startColor="#1C1124"
-        android:centerColor="#0E0A18"
-        android:endColor="#060408"/>
+        android:centerX="0.5"
+        android:centerY="0.4"
+        android:startColor="#1B2A3B"
+        android:endColor="#06090E"/>
 </shape>