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/configs/NotificationServiceConfig.dart

@@ -1,5 +1,6 @@
 import 'dart:io';
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter_local_notifications/flutter_local_notifications.dart';
 
 import '../about/external/data/Commitment.dart';
@@ -32,7 +33,7 @@ class LocalNotificationEngine {
   static bool _ready = false;
 
   static Future<void> init() async {
-    if (_ready) {
+    if (_ready || kIsWeb) {
       return;
     }
 
@@ -105,6 +106,10 @@ class LocalNotificationEngine {
   /// The ongoing notification for a running task. Shows the live remaining
   /// time so it is useful from the lock screen without unlocking.
   static Future<void> showSessionNotification(LiveSession session) async {
+    // The browser has no notification channels; the runner works without.
+    if (kIsWeb) {
+      return;
+    }
     await init();
 
     final String remaining = session.requiredSeconds > 0
@@ -144,6 +149,10 @@ class LocalNotificationEngine {
   }
 
   static Future<void> cancelSessionNotification() async {
+    // The browser has no notification channels; the runner works without.
+    if (kIsWeb) {
+      return;
+    }
     await init();
     await plugin.cancel(id: sessionNotificationId);
   }
@@ -156,6 +165,9 @@ class LocalNotificationEngine {
     EscalationTier tier,
     ToneLevel tone,
   ) async {
+    if (kIsWeb) {
+      return;
+    }
     await init();
 
     final bool nonNegotiable =
@@ -198,6 +210,10 @@ class LocalNotificationEngine {
   }
 
   static Future<void> cancelAll() async {
+    // The browser has no notification channels; the runner works without.
+    if (kIsWeb) {
+      return;
+    }
     await init();
     await plugin.cancelAll();
   }