Update wiring diagram maker: formboard router + frontend

Backend formboard router and frontend (canvas, formboard, connector library,
app/api, index/css) updates. Ignore *.log.

frontend/js/api.js

@@ -6,6 +6,7 @@ async function apiFetch(path, options = {}) {
     const text = await res.text();
     throw new Error(`API ${res.status}: ${text}`);
   }
+  if (res.status === 204) return null;
   return res.json();
 }