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/build.gradle.kts

@@ -84,6 +84,18 @@ kotlin {
     }
 }
 
+dependencies {
+    // Device sync's periodic/one-off background jobs (SyncWorker,
+    // ConflictResolveWorker) - see their doc comments for why this is
+    // plain WorkManager rather than a Dart-side background-task plugin.
+    implementation("androidx.work:work-runtime-ktx:2.9.1")
+    // PROPFIND (WebDAV directory listing) - Android's HttpURLConnection
+    // hard-rejects any method outside {OPTIONS,GET,HEAD,POST,PUT,DELETE,
+    // TRACE,PATCH} (ProtocolException), unlike plain OpenJDK. OkHttp has
+    // no such whitelist. See SyncEngine.kt.
+    implementation("com.squareup.okhttp3:okhttp:4.12.0")
+}
+
 flutter {
     source = "../.."
 }