feat: Add PWA notification support with badge counters

Implements Progressive Web App notification capabilities:

- Service Worker foundation for PWA installability and offline support
- Browser notification toggle in side menu with permission management
- Local notifications triggered on new messages (channels, DMs, pending)
- App Badge API integration showing total unread count on app icon
- Smart notification logic (only when app hidden, only for NEW messages)
- Graceful degradation for unsupported browsers

Technical details:
- Service Worker with network-first fetch strategy for dynamic content
- Notification permission stored in localStorage (mc_notifications_enabled)
- Delta-based notification tracking (prevents spam on page load)
- Notification tag prevents duplicate alerts
- App badge auto-clears when user returns to app

Limitations (documented):
- Android may freeze background JS after 5-10 minutes (OS behavior)
- Full "wake device" support requires Web Push API (future enhancement)
- Works best for active users who check app regularly

Files modified:
- app/static/js/sw.js (new)
- app/templates/base.html
- app/static/js/app.js
- app/static/js/dm.js
- app/static/manifest.json
- app/static/css/style.css

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/static/css/style.css

@@ -688,3 +688,13 @@ main {
         padding-bottom: env(safe-area-inset-bottom);
     }
 }
+
+/* =============================================================================
+   PWA Notifications
+   ============================================================================= */
+
+/* Notification toggle disabled state */
+#notificationsToggle:disabled {
+    opacity: 0.6;
+    cursor: not-allowed;
+}