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."
 }