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/index.html

@@ -7,8 +7,19 @@
   <meta name="description"
     content="Personal task management app with calendar scheduling, kanban boards, and time tracking" />
   <meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1, user-scalable=no" />
+
+  <!-- PWA Meta Tags -->
+  <link rel="manifest" href="/manifest.json" />
+  <meta name="theme-color" content="#7c3aed" />
+  <meta name="apple-mobile-web-app-capable" content="yes" />
+  <meta name="apple-mobile-web-app-status-bar-style" content="black-translucent" />
+  <meta name="apple-mobile-web-app-title" content="TaskFlow" />
+
+  <!-- Icons -->
   <link rel="icon" type="image/png" href="/favicon.png?v=4" />
   <link rel="apple-touch-icon" href="/favicon.png?v=4" />
+
+  <!-- Fonts -->
   <link rel="preconnect" href="https://fonts.googleapis.com">
   <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
   <link