Add removable mock mode for a clickable preview

Runs the whole app on seeded in-memory data with no backend or device
storage, behind useMockData in lib/Grounded/mock/MockSwitch.dart. To
remove: delete lib/Grounded/mock/ and the lines tagged "MOCK:" in
ParentViewModel (or revert this commit, keeping the notification guard).

Also skips local notifications on web, where the plugin and dart:io
Platform both throw.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KhQh614DMeYoQEH9SoMRUw

frontend/lib/Grounded/mock/MockMemory.dart

@@ -0,0 +1,26 @@
+// MOCK DATA — TEMPORARY. See MockSwitch.dart.
+
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
+
+import '../about/internal/application/MeDescription.dart';
+import '../about/internal/application/UserDetails.dart';
+import '../memory/InternalMemory.dart';
+import 'MockStorage.dart';
+
+/// The real [InternalMemory], pointed at in-memory storage and pre-seeded
+/// with a signed-in user so the app opens straight onto Home.
+class MockMemory extends InternalMemory {
+  static bool _seeded = false;
+
+  MockMemory() {
+    if (!_seeded) {
+      _seeded = true;
+      setMyDescription(
+          MeDescription(id: "mock-user", name: "Alex", token: "mock-token"));
+      setUserDetails(UserDetails(pic: "", name: "Alex"));
+    }
+  }
+
+  @override
+  FlutterSecureStorage get groundedStorage => const MockStorage();
+}