feat(vault): pick a notebook vault folder on first run

Phase 0 of the file-based storage plan (docs/plans/
2026-06-24-file-based-storage.md). Foundation only — no editor or
DB change yet.

- VaultService (SharedPreferences): stores the vault root path,
  vaultRootValid() = path set AND directory exists.
- VaultSetupScreen: first-run folder picker (file_picker, Windows).
- main.dart gates HomeScreen behind a valid vault, re-prompting if
  the saved folder is gone.
- Settings: a Vault section to change the folder.

Editors still use SQLite; later phases move annotations into
per-file sidecars under the vault. analyze clean, tests green.

lib/l10n/app_localizations_zh.dart

@@ -256,4 +256,46 @@ class AppLocalizationsZh extends AppLocalizations {
 
   @override
   String get close => '关闭';
+
+  @override
+  String get vaultSetupTitle => '选择笔记库';
+
+  @override
+  String get vaultSetupHeadline => '为你的笔记本选择一个文件夹';
+
+  @override
+  String get vaultSetupBody =>
+      'BadNote 会把你的笔记本都存放在你选择的一个文件夹里——就像 Obsidian 的库（vault）。请选择一个你能掌控的文件夹（例如同步盘），这样你的笔记会跟着文件一起走。';
+
+  @override
+  String get vaultChooseFolder => '选择文件夹';
+
+  @override
+  String get vaultMissingTitle => '笔记库文件夹不见了';
+
+  @override
+  String get vaultMissingBody => '找不到你选择的文件夹（可能被移动、删除，或所在磁盘已拔出）。请重新定位或另选一个。';
+
+  @override
+  String vaultPickFailed(String error) {
+    return '无法打开文件夹选择器：$error';
+  }
+
+  @override
+  String get vaultNotWritable => '该文件夹不可写，请另选一个。';
+
+  @override
+  String get vaultSection => '笔记库';
+
+  @override
+  String get vaultFolderLabel => '笔记库文件夹';
+
+  @override
+  String get vaultNoneSelected => '尚未选择文件夹';
+
+  @override
+  String get vaultChangeFolder => '更改笔记库文件夹';
+
+  @override
+  String get vaultUpdated => '笔记库文件夹已更新';
 }