fix: use base URL for Supabase client, nginx proxies /rest/v1/ and /auth/v1/

The Supabase JS client auto-derives API paths from the base URL:
- REST: ${url}/rest/v1/
- Auth: ${url}/auth/v1/

Nginx now proxies these standard paths to the correct backend services.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -69,9 +69,8 @@ services:
     build:
       context: .
       args:
-        VITE_SUPABASE_URL: ${SITE_URL:-http://localhost:8080}/rest
+        VITE_SUPABASE_URL: ${SITE_URL:-http://localhost:8080}
         VITE_SUPABASE_ANON_KEY: ${ANON_KEY}
-        VITE_AUTH_URL: ${SITE_URL:-http://localhost:8080}/auth
         VITE_COLLAB_WS_URL: ${COLLAB_WS_URL:-ws://localhost:8080/ws}
     restart: unless-stopped
     ports: