fix: pass NEXT_PUBLIC_* env vars as build args for Supabase client

Next.js inlines NEXT_PUBLIC_* vars at build time. They must be
available as ARGs during the Docker build, not just as runtime
environment variables.

docker-compose.yml

@@ -22,6 +22,9 @@ services:
   frontend:
     build:
       context: ./frontend
+      args:
+        NEXT_PUBLIC_SUPABASE_URL: ${SUPABASE_URL}
+        NEXT_PUBLIC_SUPABASE_ANON_KEY: ${SUPABASE_ANON_KEY}
     expose:
       - "3000"
     depends_on: