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.dart

@@ -787,6 +787,132 @@ abstract class AppLocalizations {
   /// In en, this message translates to:
   /// **'Vault folder updated'**
   String get vaultUpdated;
+
+  /// No description provided for @syncSection.
+  ///
+  /// In en, this message translates to:
+  /// **'Sync (WebDAV)'**
+  String get syncSection;
+
+  /// No description provided for @syncServerUrl.
+  ///
+  /// In en, this message translates to:
+  /// **'Server URL'**
+  String get syncServerUrl;
+
+  /// No description provided for @syncServerUrlHint.
+  ///
+  /// In en, this message translates to:
+  /// **'https://dav.example.com/remote.php/dav/files/me'**
+  String get syncServerUrlHint;
+
+  /// No description provided for @syncUsername.
+  ///
+  /// In en, this message translates to:
+  /// **'Username'**
+  String get syncUsername;
+
+  /// No description provided for @syncPassword.
+  ///
+  /// In en, this message translates to:
+  /// **'Password'**
+  String get syncPassword;
+
+  /// No description provided for @syncRemoteFolder.
+  ///
+  /// In en, this message translates to:
+  /// **'Remote folder'**
+  String get syncRemoteFolder;
+
+  /// No description provided for @syncRemoteFolderHint.
+  ///
+  /// In en, this message translates to:
+  /// **'BadNote'**
+  String get syncRemoteFolderHint;
+
+  /// No description provided for @syncSave.
+  ///
+  /// In en, this message translates to:
+  /// **'Save'**
+  String get syncSave;
+
+  /// No description provided for @syncSaved.
+  ///
+  /// In en, this message translates to:
+  /// **'Sync settings saved'**
+  String get syncSaved;
+
+  /// No description provided for @syncTestConnection.
+  ///
+  /// In en, this message translates to:
+  /// **'Test connection'**
+  String get syncTestConnection;
+
+  /// No description provided for @syncTestOk.
+  ///
+  /// In en, this message translates to:
+  /// **'Connection OK'**
+  String get syncTestOk;
+
+  /// No description provided for @syncTestFailed.
+  ///
+  /// In en, this message translates to:
+  /// **'Connection failed: {error}'**
+  String syncTestFailed(String error);
+
+  /// No description provided for @syncNow.
+  ///
+  /// In en, this message translates to:
+  /// **'Sync now'**
+  String get syncNow;
+
+  /// No description provided for @syncRunning.
+  ///
+  /// In en, this message translates to:
+  /// **'Syncing…'**
+  String get syncRunning;
+
+  /// No description provided for @syncNeverRun.
+  ///
+  /// In en, this message translates to:
+  /// **'Never synced'**
+  String get syncNeverRun;
+
+  /// No description provided for @syncLastRun.
+  ///
+  /// In en, this message translates to:
+  /// **'Last synced: {when}'**
+  String syncLastRun(String when);
+
+  /// No description provided for @syncResultSummary.
+  ///
+  /// In en, this message translates to:
+  /// **'{uploaded} uploaded · {downloaded} downloaded · {conflicts} conflicts'**
+  String syncResultSummary(int uploaded, int downloaded, int conflicts);
+
+  /// No description provided for @syncFailed.
+  ///
+  /// In en, this message translates to:
+  /// **'Sync failed: {error}'**
+  String syncFailed(String error);
+
+  /// No description provided for @syncAuto.
+  ///
+  /// In en, this message translates to:
+  /// **'Sync automatically on launch'**
+  String get syncAuto;
+
+  /// No description provided for @syncCredentialsNote.
+  ///
+  /// In en, this message translates to:
+  /// **'Credentials are stored locally in plain text. Use a dedicated app password.'**
+  String get syncCredentialsNote;
+
+  /// No description provided for @syncNotConfigured.
+  ///
+  /// In en, this message translates to:
+  /// **'Enter a server URL to enable sync.'**
+  String get syncNotConfigured;
 }
 
 class _AppLocalizationsDelegate