fix(backup): allow selecting SFLB archives for restore

lib/screens/settings/backup_restore_page.dart

@@ -74,10 +74,10 @@ class _BackupRestorePageState extends ConsumerState<BackupRestorePage> {
 
     BackupBundle? bundle;
     try {
-      final picked = await FilePicker.pickFile(
-        type: FileType.custom,
-        allowedExtensions: ['json', BackupService.fileExtension],
-      );
+      // Android resolves custom extensions to MIME types. It recognizes JSON
+      // but not SFLB, so mixing them hides our own backups. Validate contents
+      // with BackupService after selection instead of filtering by extension.
+      final picked = await FilePicker.pickFile(type: FileType.any);
       if (picked == null) return;
       final path = picked.path;
       bundle = path != null

test/backup_restore_picker_test.dart

@@ -0,0 +1,59 @@
+import 'package:file_picker/file_picker.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:spotiflac_android/l10n/app_localizations.dart';
+import 'package:spotiflac_android/screens/settings/backup_restore_page.dart';
+
+class _BackupFilePicker extends FilePickerPlatform {
+  bool opened = false;
+
+  @override
+  Future<PlatformFile?> pickFile({
+    String? dialogTitle,
+    String? initialDirectory,
+    FileType type = FileType.any,
+    List<String>? allowedExtensions,
+    dynamic Function(FilePickerStatus)? onFileLoading,
+    int compressionQuality = 0,
+    AndroidOptions androidOptions = const AndroidOptions(),
+    DarwinOptions darwinOptions = const DarwinOptions(),
+    WindowsOptions windowsOptions = const WindowsOptions(),
+    LinuxOptions linuxOptions = const LinuxOptions(),
+    WebOptions webOptions = const WebOptions(),
+  }) async {
+    opened = true;
+    // A custom JSON/SFLB filter becomes JSON-only on Android because SFLB
+    // has no system MIME mapping.
+    expect(type, FileType.any);
+    expect(allowedExtensions, isNull);
+    return null;
+  }
+}
+
+void main() {
+  testWidgets('restore does not exclude backups with an unknown MIME type', (
+    tester,
+  ) async {
+    final original = FilePickerPlatform.instance;
+    final picker = _BackupFilePicker();
+    FilePickerPlatform.instance = picker;
+    addTearDown(() => FilePickerPlatform.instance = original);
+    await tester.pumpWidget(
+      const ProviderScope(
+        child: MaterialApp(
+          localizationsDelegates: AppLocalizations.localizationsDelegates,
+          supportedLocales: AppLocalizations.supportedLocales,
+          home: BackupRestorePage(),
+        ),
+      ),
+    );
+    final restore = find.text('Choose backup file');
+    await tester.ensureVisible(restore);
+    await tester.tap(restore);
+    await tester.pumpAndSettle();
+    expect(picker.opened, isTrue);
+    expect(find.byType(AlertDialog), findsNothing);
+    expect(tester.takeException(), isNull);
+  });
+}