feat: Add Web Push notifications and multiple UX improvements

- Add PWA manifest and service worker for push notifications
- Implement VAPID key generation and push subscription management
- Add push notification API endpoints (/api/push/*)
- Add push_subscriptions table to database schema
- Update notification settings UI with push support and iOS hints
- Fix translation issue showing "{ task } created" - add missing keys
- Fix mobile sidebar visibility for iOS home screen app
- Change default task filter from "all" to "open" (excludes done tasks)
- Add "Open" filter option to show only todo + inProgress tasks

client/public/manifest.json

@@ -0,0 +1,26 @@
+{
+  "name": "TaskFlow",
+  "short_name": "TaskFlow",
+  "description": "Gamified task management with AI assistance",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#0f0f23",
+  "theme_color": "#7c3aed",
+  "orientation": "portrait-primary",
+  "icons": [
+    {
+      "src": "/favicon.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any maskable"
+    },
+    {
+      "src": "/favicon.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any maskable"
+    }
+  ],
+  "categories": ["productivity", "utilities"],
+  "prefer_related_applications": false
+}