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_en.arb

@@ -99,5 +99,21 @@
   "@boardDanglingLink": {
     "placeholders": { "target": { "type": "String" } }
   },
-  "close": "Close"
+  "close": "Close",
+  "vaultSetupTitle": "Choose your vault",
+  "vaultSetupHeadline": "Pick a folder for your notebooks",
+  "vaultSetupBody": "BadNote stores your notebooks inside one folder you choose — like an Obsidian vault. Pick a folder you control (e.g. a synced folder) so your notes travel with their files.",
+  "vaultChooseFolder": "Choose folder",
+  "vaultMissingTitle": "Your vault folder is missing",
+  "vaultMissingBody": "The folder you picked can't be found (it may have been moved, deleted, or on a drive that's unplugged). Relocate it or pick a new one.",
+  "vaultPickFailed": "Couldn't open the folder picker: {error}",
+  "@vaultPickFailed": {
+    "placeholders": { "error": { "type": "String" } }
+  },
+  "vaultNotWritable": "That folder isn't writable. Please choose another.",
+  "vaultSection": "Vault",
+  "vaultFolderLabel": "Vault folder",
+  "vaultNoneSelected": "No folder selected",
+  "vaultChangeFolder": "Change vault folder",
+  "vaultUpdated": "Vault folder updated"
 }

lib/l10n/app_localizations.dart

@@ -571,6 +571,84 @@ abstract class AppLocalizations {
   /// In en, this message translates to:
   /// **'Close'**
   String get close;
+
+  /// No description provided for @vaultSetupTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'Choose your vault'**
+  String get vaultSetupTitle;
+
+  /// No description provided for @vaultSetupHeadline.
+  ///
+  /// In en, this message translates to:
+  /// **'Pick a folder for your notebooks'**
+  String get vaultSetupHeadline;
+
+  /// No description provided for @vaultSetupBody.
+  ///
+  /// In en, this message translates to:
+  /// **'BadNote stores your notebooks inside one folder you choose — like an Obsidian vault. Pick a folder you control (e.g. a synced folder) so your notes travel with their files.'**
+  String get vaultSetupBody;
+
+  /// No description provided for @vaultChooseFolder.
+  ///
+  /// In en, this message translates to:
+  /// **'Choose folder'**
+  String get vaultChooseFolder;
+
+  /// No description provided for @vaultMissingTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'Your vault folder is missing'**
+  String get vaultMissingTitle;
+
+  /// No description provided for @vaultMissingBody.
+  ///
+  /// In en, this message translates to:
+  /// **'The folder you picked can\'t be found (it may have been moved, deleted, or on a drive that\'s unplugged). Relocate it or pick a new one.'**
+  String get vaultMissingBody;
+
+  /// No description provided for @vaultPickFailed.
+  ///
+  /// In en, this message translates to:
+  /// **'Couldn\'t open the folder picker: {error}'**
+  String vaultPickFailed(String error);
+
+  /// No description provided for @vaultNotWritable.
+  ///
+  /// In en, this message translates to:
+  /// **'That folder isn\'t writable. Please choose another.'**
+  String get vaultNotWritable;
+
+  /// No description provided for @vaultSection.
+  ///
+  /// In en, this message translates to:
+  /// **'Vault'**
+  String get vaultSection;
+
+  /// No description provided for @vaultFolderLabel.
+  ///
+  /// In en, this message translates to:
+  /// **'Vault folder'**
+  String get vaultFolderLabel;
+
+  /// No description provided for @vaultNoneSelected.
+  ///
+  /// In en, this message translates to:
+  /// **'No folder selected'**
+  String get vaultNoneSelected;
+
+  /// No description provided for @vaultChangeFolder.
+  ///
+  /// In en, this message translates to:
+  /// **'Change vault folder'**
+  String get vaultChangeFolder;
+
+  /// No description provided for @vaultUpdated.
+  ///
+  /// In en, this message translates to:
+  /// **'Vault folder updated'**
+  String get vaultUpdated;
 }
 
 class _AppLocalizationsDelegate

lib/l10n/app_localizations_en.dart

@@ -257,4 +257,48 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get close => 'Close';
+
+  @override
+  String get vaultSetupTitle => 'Choose your vault';
+
+  @override
+  String get vaultSetupHeadline => 'Pick a folder for your notebooks';
+
+  @override
+  String get vaultSetupBody =>
+      'BadNote stores your notebooks inside one folder you choose — like an Obsidian vault. Pick a folder you control (e.g. a synced folder) so your notes travel with their files.';
+
+  @override
+  String get vaultChooseFolder => 'Choose folder';
+
+  @override
+  String get vaultMissingTitle => 'Your vault folder is missing';
+
+  @override
+  String get vaultMissingBody =>
+      'The folder you picked can\'t be found (it may have been moved, deleted, or on a drive that\'s unplugged). Relocate it or pick a new one.';
+
+  @override
+  String vaultPickFailed(String error) {
+    return 'Couldn\'t open the folder picker: $error';
+  }
+
+  @override
+  String get vaultNotWritable =>
+      'That folder isn\'t writable. Please choose another.';
+
+  @override
+  String get vaultSection => 'Vault';
+
+  @override
+  String get vaultFolderLabel => 'Vault folder';
+
+  @override
+  String get vaultNoneSelected => 'No folder selected';
+
+  @override
+  String get vaultChangeFolder => 'Change vault folder';
+
+  @override
+  String get vaultUpdated => 'Vault folder updated';
 }

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';
 }

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"
 }