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"
 }