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_en.arb

@@ -150,5 +150,42 @@
   "vaultFolderLabel": "Vault folder",
   "vaultNoneSelected": "No folder selected",
   "vaultChangeFolder": "Change vault folder",
-  "vaultUpdated": "Vault folder updated"
+  "vaultUpdated": "Vault folder updated",
+  "syncSection": "Sync (WebDAV)",
+  "syncServerUrl": "Server URL",
+  "syncServerUrlHint": "https://dav.example.com/remote.php/dav/files/me",
+  "syncUsername": "Username",
+  "syncPassword": "Password",
+  "syncRemoteFolder": "Remote folder",
+  "syncRemoteFolderHint": "BadNote",
+  "syncSave": "Save",
+  "syncSaved": "Sync settings saved",
+  "syncTestConnection": "Test connection",
+  "syncTestOk": "Connection OK",
+  "syncTestFailed": "Connection failed: {error}",
+  "@syncTestFailed": {
+    "placeholders": { "error": { "type": "String" } }
+  },
+  "syncNow": "Sync now",
+  "syncRunning": "Syncing…",
+  "syncNeverRun": "Never synced",
+  "syncLastRun": "Last synced: {when}",
+  "@syncLastRun": {
+    "placeholders": { "when": { "type": "String" } }
+  },
+  "syncResultSummary": "{uploaded} uploaded · {downloaded} downloaded · {conflicts} conflicts",
+  "@syncResultSummary": {
+    "placeholders": {
+      "uploaded": { "type": "int" },
+      "downloaded": { "type": "int" },
+      "conflicts": { "type": "int" }
+    }
+  },
+  "syncFailed": "Sync failed: {error}",
+  "@syncFailed": {
+    "placeholders": { "error": { "type": "String" } }
+  },
+  "syncAuto": "Sync automatically on launch",
+  "syncCredentialsNote": "Credentials are stored locally in plain text. Use a dedicated app password.",
+  "syncNotConfigured": "Enter a server URL to enable sync."
 }

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

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.';
 }

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。';
 }

lib/l10n/app_zh.arb

@@ -120,5 +120,42 @@
   "vaultFolderLabel": "笔记库文件夹",
   "vaultNoneSelected": "尚未选择文件夹",
   "vaultChangeFolder": "更改笔记库文件夹",
-  "vaultUpdated": "笔记库文件夹已更新"
+  "vaultUpdated": "笔记库文件夹已更新",
+  "syncSection": "同步（WebDAV）",
+  "syncServerUrl": "服务器地址",
+  "syncServerUrlHint": "https://dav.example.com/remote.php/dav/files/me",
+  "syncUsername": "用户名",
+  "syncPassword": "密码",
+  "syncRemoteFolder": "远程文件夹",
+  "syncRemoteFolderHint": "BadNote",
+  "syncSave": "保存",
+  "syncSaved": "同步设置已保存",
+  "syncTestConnection": "测试连接",
+  "syncTestOk": "连接成功",
+  "syncTestFailed": "连接失败：{error}",
+  "@syncTestFailed": {
+    "placeholders": { "error": { "type": "String" } }
+  },
+  "syncNow": "立即同步",
+  "syncRunning": "同步中…",
+  "syncNeverRun": "尚未同步",
+  "syncLastRun": "上次同步：{when}",
+  "@syncLastRun": {
+    "placeholders": { "when": { "type": "String" } }
+  },
+  "syncResultSummary": "上传 {uploaded} · 下载 {downloaded} · 冲突 {conflicts}",
+  "@syncResultSummary": {
+    "placeholders": {
+      "uploaded": { "type": "int" },
+      "downloaded": { "type": "int" },
+      "conflicts": { "type": "int" }
+    }
+  },
+  "syncFailed": "同步失败：{error}",
+  "@syncFailed": {
+    "placeholders": { "error": { "type": "String" } }
+  },
+  "syncAuto": "启动时自动同步",
+  "syncCredentialsNote": "凭据以明文保存在本地，建议使用专用的应用密码。",
+  "syncNotConfigured": "请输入服务器地址以启用同步。"
 }