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.

Docs/Web-Development.md

@@ -130,10 +130,15 @@ frontend/
 ├── .eslintrc.cjs
 ├── .prettierrc
 ├── tsconfig.json
-├── vite.config.ts
+├── vite.config.ts           # Dev proxy: /api → http://backend:8000 (service DNS)
 └── package.json
 ```
 
+> **Dev proxy target:** `vite.config.ts` proxies all `/api` requests to
+> `http://backend:8000`.  Use the compose **service name** (`backend`), not
+> `localhost` — inside the container network `localhost` resolves to the
+> frontend container itself and causes `ECONNREFUSED`.
+
 ### Separation of Concerns
 
 - **Pages** handle routing and compose layout + components — they contain no business logic.