fix(test): stop the desktop prefs preflight deadlocking widget tests on Linux

PrefsRecovery.assertStoreReadable reads the real on-disk store through
path_provider, and on the hosts where it is active (Linux and Windows)
that genuine file IO can never complete inside testWidgets' fake async
zone. Since 7f0cad33 wired the preflight into shared-preference init,
every widget test that initialises settings on the Linux CI runner hung
for its full ten-minute timeout, turning the unit-test job into a
six-hour cancellation. The hermetic prefs fixture swaps the backends for
in-memory fakes anyway, so it now disables the preflight and restores it
on teardown; the repair-flow suite keeps opting in explicitly.

Verified in an ubuntu-24.04 container on Flutter 3.44.0: the full suite
now completes in nine minutes with zero failures.

test/test_helpers/prefs.dart

@@ -1,5 +1,6 @@
 import 'package:flutter_test/flutter_test.dart';
 import 'package:plezy/services/base_shared_preferences_service.dart';
+import 'package:plezy/services/prefs_recovery.dart';
 import 'package:plezy/services/settings_service.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 import 'package:shared_preferences_platform_interface/in_memory_shared_preferences_async.dart';
@@ -17,8 +18,18 @@ void resetSharedPreferencesForTest({Map<String, Object> initialAsync = const {}}
     SharedPreferences.resetStatic();
     SettingsService.resetForTesting();
     BaseSharedPreferencesService.resetForTesting();
+    PrefsRecovery.debugSetSupportedPlatformOverride(null);
   });
   TestWidgetsFlutterBinding.ensureInitialized();
+  // The desktop store preflight (`PrefsRecovery.assertStoreReadable`) reads
+  // the real on-disk store through path_provider. These fixtures replace the
+  // preference backends with in-memory fakes, so there is no real store to
+  // validate — and on the hosts where the preflight is active (Linux and
+  // Windows) its genuine file IO can never complete inside `testWidgets`'
+  // fake async zone, deadlocking every widget test that initialises settings
+  // until the ten-minute timeout. Suites that exercise the preflight itself
+  // (`prefs_store_repair_flow_test.dart`) opt back in explicitly.
+  PrefsRecovery.debugSetSupportedPlatformOverride(false);
   SharedPreferences.setMockInitialValues({});
   SharedPreferencesAsyncPlatform.instance = initialAsync.isEmpty
       ? InMemorySharedPreferencesAsync.empty()