site: dawn.gg-style landing at root + port /skin/* for TG bot

- serve new launcher landing site (zern.cc root -> launcher server 1582)
- static assets mounted at /css /js /img
- port GET/HEAD /skin/{filename} from old site with identical headers
- exempt /skin/* + site assets from rate-limit and cache middleware
- Caddy: root domains -> 1582, old site preserved at legacy.zernmc.*

server/site/img/zernmc.svg

@@ -0,0 +1,10 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="256" height="256" viewBox="0 0 256 256">
+  <defs>
+    <linearGradient id="g" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0%" stop-color="#e94560"/>
+      <stop offset="100%" stop-color="#ff6b6b"/>
+    </linearGradient>
+  </defs>
+  <rect width="256" height="256" rx="64" fill="url(#g)"/>
+  <path d="M82 128 L128 82 L174 128 L128 174 Z" fill="white" opacity="0.9"/>
+</svg>