Use dev port as default fallback in vite-plugin-fastapi, instead of default port. No effect on invocations via devserver script.

template/frontend/vite-plugin-fastapi.js

@@ -11,7 +11,7 @@
  */
 
 export default function fastapiVue({ paths = ["/api"] } = {}) {
-  const backendUrl = process.env.ENVPREFIX_BACKEND_URL || "http://localhost:TEMPLATE_DEFAULT_PORT"
+  const backendUrl = process.env.ENVPREFIX_BACKEND_URL || "http://localhost:TEMPLATE_DEV_PORT"
 
   // Build proxy configuration for each path
   const proxy = {}