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>

Dockerfile

@@ -33,11 +33,9 @@ WORKDIR /app/calcpad-web
 # Build-time env vars for Vite (inlined at build time)
 ARG VITE_SUPABASE_URL
 ARG VITE_SUPABASE_ANON_KEY
-ARG VITE_AUTH_URL
 ARG VITE_COLLAB_WS_URL
 ENV VITE_SUPABASE_URL=$VITE_SUPABASE_URL
 ENV VITE_SUPABASE_ANON_KEY=$VITE_SUPABASE_ANON_KEY
-ENV VITE_AUTH_URL=$VITE_AUTH_URL
 ENV VITE_COLLAB_WS_URL=$VITE_COLLAB_WS_URL
 
 # Install dependencies first (layer caching)