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_zh.dart

@@ -376,4 +376,76 @@ class AppLocalizationsZh extends AppLocalizations {
 
   @override
   String get vaultUpdated => '笔记库文件夹已更新';
+
+  @override
+  String get syncSection => '同步（WebDAV）';
+
+  @override
+  String get syncServerUrl => '服务器地址';
+
+  @override
+  String get syncServerUrlHint =>
+      'https://dav.example.com/remote.php/dav/files/me';
+
+  @override
+  String get syncUsername => '用户名';
+
+  @override
+  String get syncPassword => '密码';
+
+  @override
+  String get syncRemoteFolder => '远程文件夹';
+
+  @override
+  String get syncRemoteFolderHint => 'BadNote';
+
+  @override
+  String get syncSave => '保存';
+
+  @override
+  String get syncSaved => '同步设置已保存';
+
+  @override
+  String get syncTestConnection => '测试连接';
+
+  @override
+  String get syncTestOk => '连接成功';
+
+  @override
+  String syncTestFailed(String error) {
+    return '连接失败：$error';
+  }
+
+  @override
+  String get syncNow => '立即同步';
+
+  @override
+  String get syncRunning => '同步中…';
+
+  @override
+  String get syncNeverRun => '尚未同步';
+
+  @override
+  String syncLastRun(String when) {
+    return '上次同步：$when';
+  }
+
+  @override
+  String syncResultSummary(int uploaded, int downloaded, int conflicts) {
+    return '上传 $uploaded · 下载 $downloaded · 冲突 $conflicts';
+  }
+
+  @override
+  String syncFailed(String error) {
+    return '同步失败：$error';
+  }
+
+  @override
+  String get syncAuto => '启动时自动同步';
+
+  @override
+  String get syncCredentialsNote => '凭据以明文保存在本地，建议使用专用的应用密码。';
+
+  @override
+  String get syncNotConfigured => '请输入服务器地址以启用同步。';
 }