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/MockStorage.dart

@@ -0,0 +1,55 @@
+// MOCK DATA — TEMPORARY. See MockSwitch.dart.
+
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
+
+/// Secure storage held in memory, so mock mode never touches the keychain or
+/// browser storage. Seeded as signed in.
+class MockStorage extends FlutterSecureStorage {
+  static final Map<String, String> _values = <String, String>{};
+
+  const MockStorage();
+
+  @override
+  Future<String?> read({
+    required String key,
+    AppleOptions? iOptions,
+    AndroidOptions? aOptions,
+    LinuxOptions? lOptions,
+    WebOptions? webOptions,
+    AppleOptions? mOptions,
+    WindowsOptions? wOptions,
+  }) async {
+    return _values[key];
+  }
+
+  @override
+  Future<void> write({
+    required String key,
+    required String? value,
+    AppleOptions? iOptions,
+    AndroidOptions? aOptions,
+    LinuxOptions? lOptions,
+    WebOptions? webOptions,
+    AppleOptions? mOptions,
+    WindowsOptions? wOptions,
+  }) async {
+    if (value == null) {
+      _values.remove(key);
+      return;
+    }
+    _values[key] = value;
+  }
+
+  @override
+  Future<void> delete({
+    required String key,
+    AppleOptions? iOptions,
+    AndroidOptions? aOptions,
+    LinuxOptions? lOptions,
+    WebOptions? webOptions,
+    AppleOptions? mOptions,
+    WindowsOptions? wOptions,
+  }) async {
+    _values.remove(key);
+  }
+}