fix(ios): create security-scoped bookmark inside native folder picker session

The download/library folder picker on iOS previously took the bare path
string returned by file_picker and rebuilt a URL from it to create a
security-scoped bookmark. By then the picker's access grant is gone, so
bookmark creation either failed (folder selection silently rejected) or
produced a bookmark that could not be re-opened, after which the first
download run overwrote the user's chosen folder back to app Documents.

- Add a native pickIosDirectory method channel that presents
  UIDocumentPickerViewController itself and creates the bookmark inside
  the delegate callback, while the security-scoped grant is active
- Use it for the download directory (settings + first-run setup) and
  the local library folder pickers
- Stop overwriting the saved download directory when the bookmark fails
  at queue start; fail queued items with a clear message instead
- Skip the launch-time iOS path normalizer when a bookmark exists, so
  it no longer rewrites external folder paths and drops their bookmark

Fixes #454

lib/services/platform_bridge.dart

@@ -22,6 +22,15 @@ class ExtensionSessionGrantEvent {
   });
 }
 
+/// Folder picked via the native iOS document picker: the filesystem path plus
+/// the security-scoped bookmark that re-grants access across app launches.
+class IosPickedDirectory {
+  final String path;
+  final String bookmark;
+
+  const IosPickedDirectory({required this.path, required this.bookmark});
+}
+
 class _BridgeCacheEntry {
   final Map<String, dynamic> value;
   final DateTime expiresAt;
@@ -2041,6 +2050,20 @@ class PlatformBridge {
     return const <String, dynamic>{};
   }
 
+  /// Present the native iOS folder picker. The security-scoped bookmark is
+  /// created inside the picker callback while its access grant is still
+  /// active — creating it later from a bare path loses the grant.
+  /// Returns null when the user cancels; throws on picker/bookmark failure.
+  static Future<IosPickedDirectory?> pickIosDirectory() async {
+    final result = await _channel.invokeMethod('pickIosDirectory');
+    final map = _decodeNullableMapResult(result, 'pickIosDirectory');
+    if (map == null) return null;
+    final path = map['path'] as String? ?? '';
+    final bookmark = map['bookmark'] as String? ?? '';
+    if (path.isEmpty || bookmark.isEmpty) return null;
+    return IosPickedDirectory(path: path, bookmark: bookmark);
+  }
+
   /// Create a security-scoped bookmark from a filesystem path picked by
   /// FilePicker on iOS. Must be called while the picker session is still active.
   /// Returns base64-encoded bookmark data, or null on failure.