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

@@ -0,0 +1,30 @@
+{
+  "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"
+}