Add PWA infrastructure for iOS notification support

- Add manifest.json for proper PWA installation
- Add service worker (sw.js) for push notification handling
- Add ServiceWorkerProvider to register SW and manage notifications
- Generate PNG icons (192x192, 512x512) for iOS compatibility
- Update settings to show notification status and test button
- Use service worker showNotification for iOS Safari compatibility

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

public/manifest.json

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+{
+  "name": "Quiet Thanks",
+  "short_name": "Quiet Thanks",
+  "description": "A calm, private gratitude and mood log",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#0a0a0a",
+  "theme_color": "#0a0a0a",
+  "orientation": "portrait-primary",
+  "icons": [
+    {
+      "src": "/icons/icon.svg",
+      "sizes": "any",
+      "type": "image/svg+xml",
+      "purpose": "any maskable"
+    },
+    {
+      "src": "/icons/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png"
+    },
+    {
+      "src": "/icons/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png"
+    }
+  ],
+  "categories": ["lifestyle", "health"],
+  "id": "quiet-thanks-pwa"
+}

public/sw.js

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+// Service Worker for Quiet Thanks PWA
+const CACHE_NAME = 'quiet-thanks-v1';
+
+// Install event - cache essential assets
+self.addEventListener('install', (event) => {
+  console.log('[SW] Installing service worker...');
+  self.skipWaiting();
+});
+
+// Activate event - clean up old caches
+self.addEventListener('activate', (event) => {
+  console.log('[SW] Activating service worker...');
+  event.waitUntil(clients.claim());
+});
+
+// Push event - handle incoming push notifications
+self.addEventListener('push', (event) => {
+  console.log('[SW] Push event received');
+
+  let data = {
+    title: 'Quiet Thanks',
+    body: 'Take a moment to reflect on what you\'re grateful for today.',
+    icon: '/icons/icon.svg',
+    badge: '/icons/icon.svg',
+    tag: 'daily-reminder',
+  };
+
+  if (event.data) {
+    try {
+      const payload = event.data.json();
+      data = { ...data, ...payload };
+    } catch (e) {
+      data.body = event.data.text();
+    }
+  }
+
+  const options = {
+    body: data.body,
+    icon: data.icon,
+    badge: data.badge,
+    tag: data.tag,
+    vibrate: [100, 50, 100],
+    data: {
+      url: '/',
+    },
+    actions: [
+      { action: 'open', title: 'Open app' },
+      { action: 'dismiss', title: 'Dismiss' },
+    ],
+  };
+
+  event.waitUntil(
+    self.registration.showNotification(data.title, options)
+  );
+});
+
+// Notification click handler
+self.addEventListener('notificationclick', (event) => {
+  console.log('[SW] Notification clicked');
+  event.notification.close();
+
+  if (event.action === 'dismiss') {
+    return;
+  }
+
+  event.waitUntil(
+    clients.matchAll({ type: 'window', includeUncontrolled: true })
+      .then((clientList) => {
+        // Focus existing window if available
+        for (const client of clientList) {
+          if (client.url.includes(self.registration.scope) && 'focus' in client) {
+            return client.focus();
+          }
+        }
+        // Open new window
+        if (clients.openWindow) {
+          return clients.openWindow('/');
+        }
+      })
+  );
+});
+
+// Periodic sync for background reminders (when supported)
+self.addEventListener('periodicsync', (event) => {
+  if (event.tag === 'daily-reminder') {
+    event.waitUntil(checkAndShowReminder());
+  }
+});
+
+async function checkAndShowReminder() {
+  // This would check the reminder time and show notification if appropriate
+  // For now, we rely on the client-side scheduler
+  console.log('[SW] Periodic sync triggered');
+}