feat: Add presence heartbeat for Matrix online status

- matrix-gateway: POST /internal/matrix/presence/online endpoint
- usePresenceHeartbeat hook with activity tracking
- Auto away after 5 min inactivity
- Offline on page close/visibility change
- Integrated in MatrixChatRoom component

nginx/frontend.conf

@@ -0,0 +1,34 @@
+# ============================================================================
+# Frontend NGINX Config (inside frontend container)
+# ============================================================================
+
+server {
+    listen 80;
+    server_name localhost;
+    root /usr/share/nginx/html;
+    index index.html;
+    
+    # SPA fallback - all routes go to index.html
+    location / {
+        try_files $uri $uri/ /index.html;
+    }
+    
+    # Cache static assets
+    location ~* \.(js|css|png|jpg|jpeg|gif|svg|ico|woff|woff2|ttf|eot)$ {
+        expires 1y;
+        add_header Cache-Control "public, immutable";
+    }
+    
+    # Health check
+    location /health {
+        access_log off;
+        return 200 "healthy\n";
+        add_header Content-Type text/plain;
+    }
+    
+    # Disable cache for index.html
+    location = /index.html {
+        add_header Cache-Control "no-cache, no-store, must-revalidate";
+    }
+}
+