Fix API URL config: use Next.js rewrite proxy instead of NEXT_PUBLIC_*

NEXT_PUBLIC_* is baked at build time so runtime docker-compose environment
vars have no effect. Instead:
- next.config.ts rewrites /api/* -> API_BASE_URL/api/* at the server level
- api.ts uses relative paths (no env var in client bundle at all)
- docker-compose passes API_BASE_URL=http://cvfs-backend:8080 at runtime
- Dockerfile no longer needs any build args

Requests: browser -> Next.js server (/api/*) -> cvfs-backend:8080 (/api/*)

https://claude.ai/code/session_01Xmxm2QLgFBgRJyYD6VukR6

docker-compose.yml

@@ -11,8 +11,8 @@ services:
     build:
       context: ./
       dockerfile: ./docker/webapp.Dockerfile
-      args:
-        - NEXT_PUBLIC_API_BASE_URL=https://api.cv.alves.world
+    environment:
+      - API_BASE_URL=http://cvfs-backend:8080
     networks:
       - dokploy-network
       - cvfs-network