refactor: use Vite built-in DEV flag instead of VITE_OAUTH_ENABLED

import.meta.env.DEV is already true during `bun run dev` and false in
production builds, so a separate env var is unnecessary.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

opal-web/.env.example

@@ -2,5 +2,4 @@
 VITE_API_URL=https://opal.example.com/api
 VITE_AUTH_URL=https://auth.example.com
 
-# OAuth — set to "false" for local dev (used with `opal server start --dev`)
-VITE_OAUTH_ENABLED=true
+# OAuth (not needed for local dev — Vite's DEV mode auto-skips auth)