Add API proxy configuration and large favicon

- Configure Caddy to proxy /api requests to backend service
- Add favicon_big.png for various icon size needs

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

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

Caddyfile

@@ -5,6 +5,10 @@
 }
 
 :80, :443 {
+    @backend {
+        path /api /api/*
+    }
+    reverse_proxy @backend calendar-backend:3000
     try_files {path} /index.html
     root * /srv/www
     file_server