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>

lib/widgets/sync_status_badge.dart

@@ -0,0 +1,34 @@
+import 'package:flutter/material.dart';
+import '../models/sync_status.dart';
+
+/// A small corner badge over a thumbnail showing device-sync status - a
+/// `cloud_done` badge once a file's mirrored locally, `sync` while it's
+/// actively being transferred. Nothing is drawn for `none`/`conflict`
+/// (conflicts surface in the sync header, not as per-tile noise).
+class SyncStatusBadge extends StatelessWidget {
+  final SyncItemStatus status;
+
+  const SyncStatusBadge({super.key, required this.status});
+
+  @override
+  Widget build(BuildContext context) {
+    if (status != SyncItemStatus.syncing && status != SyncItemStatus.synced) {
+      return const SizedBox.shrink();
+    }
+    final colorScheme = Theme.of(context).colorScheme;
+    final icon = status == SyncItemStatus.syncing
+        ? Icons.sync_rounded
+        : Icons.cloud_done_rounded;
+
+    return Container(
+      width: 18,
+      height: 18,
+      decoration: BoxDecoration(
+        color: colorScheme.surface,
+        shape: BoxShape.circle,
+        border: Border.all(color: colorScheme.surface, width: 1.5),
+      ),
+      child: Icon(icon, size: 13, color: colorScheme.primary),
+    );
+  }
+}