fix(server): respond to HEAD / so health checks return 200

curl -sSI and Traefik probes use HEAD; the server only handled GET,
causing HEAD / to fall through to the 404 handler.

server.mjs

@@ -193,10 +193,10 @@ const server = createServer(async (req, res) => {
   const { id, session } = getSession(req.headers["cookie"]);
   const url = req.url.split("?")[0];
 
-  if (req.method === "GET" && url === "/") {
+  if ((req.method === "GET" || req.method === "HEAD") && url === "/") {
     setCookie(res, id);
     res.writeHead(200, { "Content-Type": "text/html; charset=utf-8" });
-    res.end(renderPage(session, "", null));
+    res.end(req.method === "HEAD" ? "" : renderPage(session, "", null));
     return;
   }