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>

calcpad-web/src/auth/supabase.ts

@@ -2,11 +2,14 @@ import { createClient } from '@supabase/supabase-js'
 
 const supabaseUrl = import.meta.env.VITE_SUPABASE_URL as string
 const supabaseAnonKey = import.meta.env.VITE_SUPABASE_ANON_KEY as string
-const authUrl = import.meta.env.VITE_AUTH_URL as string | undefined
 
 /**
  * Supabase client singleton.
  * Returns null if env vars are not configured (local-only mode).
+ *
+ * The client auto-derives API paths from the base URL:
+ *   - REST: ${supabaseUrl}/rest/v1/
+ *   - Auth: ${supabaseUrl}/auth/v1/
  */
 export const supabase =
   supabaseUrl && supabaseAnonKey
@@ -15,7 +18,6 @@ export const supabase =
           persistSession: true,
           autoRefreshToken: true,
           detectSessionInUrl: true,
-          ...(authUrl ? { url: authUrl } : {}),
         },
       })
     : null