feat(sync): WebDAV vault sync

Two-way sync of the vault folder to a user-configured WebDAV server,
so annotations (which travel with the file) sync with the file.

- WebDavSyncService.syncNow: per-file decision — local-only uploads,
  remote-only downloads, and when BOTH sides changed since the last
  sync it keeps the loser as <file>.conflict-<mtime> on both sides
  (last-write-wins by mtime) so no data is ever lost. Creates dirs as
  needed; deletes are conservative.
- The decision logic is pure and unit-tested against a fake WebDAV
  client; the real client is a thin http adapter (no dio dependency).
- Settings: WebDAV URL / user / password / remote folder, Test
  connection, Sync now (with status + last-synced), and an auto-sync
  toggle (default OFF).

Real server round-trips are device/server-validated. Credentials are in
SharedPreferences for now (TODO secure-storage). analyze clean, 432 tests.

lib/l10n/app_localizations_en.dart

@@ -379,4 +379,77 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get vaultUpdated => 'Vault folder updated';
+
+  @override
+  String get syncSection => 'Sync (WebDAV)';
+
+  @override
+  String get syncServerUrl => 'Server URL';
+
+  @override
+  String get syncServerUrlHint =>
+      'https://dav.example.com/remote.php/dav/files/me';
+
+  @override
+  String get syncUsername => 'Username';
+
+  @override
+  String get syncPassword => 'Password';
+
+  @override
+  String get syncRemoteFolder => 'Remote folder';
+
+  @override
+  String get syncRemoteFolderHint => 'BadNote';
+
+  @override
+  String get syncSave => 'Save';
+
+  @override
+  String get syncSaved => 'Sync settings saved';
+
+  @override
+  String get syncTestConnection => 'Test connection';
+
+  @override
+  String get syncTestOk => 'Connection OK';
+
+  @override
+  String syncTestFailed(String error) {
+    return 'Connection failed: $error';
+  }
+
+  @override
+  String get syncNow => 'Sync now';
+
+  @override
+  String get syncRunning => 'Syncing…';
+
+  @override
+  String get syncNeverRun => 'Never synced';
+
+  @override
+  String syncLastRun(String when) {
+    return 'Last synced: $when';
+  }
+
+  @override
+  String syncResultSummary(int uploaded, int downloaded, int conflicts) {
+    return '$uploaded uploaded · $downloaded downloaded · $conflicts conflicts';
+  }
+
+  @override
+  String syncFailed(String error) {
+    return 'Sync failed: $error';
+  }
+
+  @override
+  String get syncAuto => 'Sync automatically on launch';
+
+  @override
+  String get syncCredentialsNote =>
+      'Credentials are stored locally in plain text. Use a dedicated app password.';
+
+  @override
+  String get syncNotConfigured => 'Enter a server URL to enable sync.';
 }