Add device sync: mirror folders/files locally, background WorkManager engine

- New native sync engine (SyncEngine/SyncWorker/ConflictResolveWorker,
  WorkManager-based) that mirrors selected Nextcloud folders or files to
  app-private local storage, periodically and on-demand ("Sync now"),
  with never-auto-resolved conflict notifications.
- Single files (not just folders) can now be marked "Sync to device".
- Live sync status (syncing/synced/conflicts) pushes from native to Dart
  over a new EventChannel; the persistent header chip/panel now reflects
  device-sync status instead of the WebDAV-refresh loading state, with a
  cloud_off/cloud_sync/cloud_done/cloud_alert icon set and an expandable
  conflicts list with in-app "Keep local"/"Use server" resolution.
  Per-item cloud_done/sync badges show on Files tiles.
- Share/Download short-circuit to the local copy for already-synced
  files instead of a fresh network fetch.
- Settings gained a Device Sync section (per-folder list, "Sync
  everything", Wi-Fi-only toggle, manual "Sync now").
- Fixed two release-only bugs found via on-device testing: Android's
  HttpURLConnection silently rejects the PROPFIND method (switched to
  OkHttp), and R8 was stripping WorkManager's reflection-instantiated
  internals (broadened proguard-rules.pro to keep androidx.work.**
  wholesale rather than chasing individual classes).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

android/app/src/main/AndroidManifest.xml

@@ -76,6 +76,13 @@
             android:name=".DownloadService"
             android:exported="false"
             android:foregroundServiceType="dataSync"/>
+        <!-- The "Keep local"/"Use server" actions on a sync-conflict
+             notification (SyncWorker/SyncEngine) - must be a manifest-
+             registered receiver, not a runtime-registered one, since it
+             needs to work even when the app process isn't running. -->
+        <receiver
+            android:name=".SyncConflictReceiver"
+            android:exported="false"/>
         <!-- Hands back the file(s) a caller picked via the GET_CONTENT filter
              above - only the app's own cache/picker/ subfolder is exposed,
              see file_paths.xml. -->