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/templates/base.html

@@ -63,6 +63,19 @@
                     <i class="bi bi-broadcast-pin" style="font-size: 1.5rem;"></i>
                     <span>Manage Channels</span>
                 </button>
+                <!-- Notifications Toggle -->
+                <button id="notificationsToggle" class="list-group-item list-group-item-action d-flex align-items-center gap-3" type="button">
+                    <i class="bi bi-bell" style="font-size: 1.5rem;"></i>
+                    <div class="flex-grow-1">
+                        <div class="d-flex justify-content-between align-items-center">
+                            <span>Powiadomienia</span>
+                            <span id="notificationStatus" class="badge bg-secondary">Wyłączone</span>
+                        </div>
+                        <small class="text-muted d-block mt-1" style="font-size: 0.75rem;">
+                            Działa gdy app w tle
+                        </small>
+                    </div>
+                </button>
                 <div class="list-group-item">
                     <div class="d-flex align-items-center gap-3 mb-2">
                         <i class="bi bi-calendar3" style="font-size: 1.5rem;"></i>
@@ -277,6 +290,22 @@
         });
     </script>
 
+    <!-- Service Worker Registration for PWA -->
+    <script>
+        // Register Service Worker for PWA functionality
+        if ('serviceWorker' in navigator) {
+            window.addEventListener('load', () => {
+                navigator.serviceWorker.register('/static/js/sw.js')
+                    .then((registration) => {
+                        console.log('Service Worker registered:', registration.scope);
+                    })
+                    .catch((error) => {
+                        console.error('Service Worker registration failed:', error);
+                    });
+            });
+        }
+    </script>
+
     {% block extra_scripts %}{% endblock %}
 </body>
 </html>