refactor: utilize Vercel Build Output API to reimplement API proxy

This replaces Vercel Edge Middleware with the Vercel Build Output
API[^1] to reimplement the API proxy, cutting costs (no more
invocation-based charges), reducing latency (no middle layer or cold
starts), and simplifying the setup, while retaining dynamic
configuration via the `VERCEL_PROXIED_API_BASE_URL` environment
variable.

Updates goplus#804

[^1]: https://vercel.com/docs/build-output-api/v3

Signed-off-by: Aofei Sheng <aofei@aofeisheng.com>

spx-gui/.env

@@ -13,6 +13,6 @@ VITE_API_BASE_URL="/api"
 # Features control
 VITE_DISABLE_AIGC="false"
 
-# This base URL is used by Vercel Edge Middleware to proxy all '/api/(.*)' requests.
+# This base URL is used by Vercel to proxy all '/api/(.*)' requests.
 # The path prefix '/api' is stripped before forwarding.
 VERCEL_PROXIED_API_BASE_URL="https://goplus-builder.qiniu.io/api"

spx-gui/.gitignore

@@ -41,3 +41,4 @@ tsconfig.tsbuildinfo
 !.yarn/versions
 
 .env.local
+.vercel