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_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。"
 }