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_zh.arb

@@ -78,5 +78,18 @@
   "boardBacklinks": "哪些链接到这里",
   "boardNoBacklinks": "暂无其他便利贴链接到这里",
   "boardDanglingLink": "没有名为“{target}”的便利贴",
-  "close": "关闭"
+  "close": "关闭",
+  "vaultSetupTitle": "选择笔记库",
+  "vaultSetupHeadline": "为你的笔记本选择一个文件夹",
+  "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。",
+  "vaultChooseFolder": "选择文件夹",
+  "vaultMissingTitle": "笔记库文件夹不见了",
+  "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。",
+  "vaultPickFailed": "无法打开文件夹选择器：{error}",
+  "vaultNotWritable": "该文件夹不可写，请另选一个。",
+  "vaultSection": "笔记库",
+  "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"
 }