fix: use backend service name in Vite proxy target

Vite runs inside the frontend container where 'localhost' resolves to
the container itself, not the backend.  Change the /api proxy target
from http://localhost:8000 to http://backend:8000 so the request is
routed to the backend service over the compose network.

frontend/vite.config.ts

@@ -14,7 +14,10 @@ export default defineConfig({
     port: 5173,
     proxy: {
       "/api": {
-        target: "http://localhost:8000",
+        // In the dev compose stack the backend is reachable via its service
+        // name on the shared Docker/Podman network.  Using "localhost" would
+        // resolve to the frontend container itself and cause ECONNREFUSED.
+        target: "http://backend:8000",
         changeOrigin: true,
       },
     },