Three changes that keep getting lost on nuke+rebuild:
1. middleware.ts: read CSP_DEV_MODE env to relax CSP in local Docker
2. api.ts: send NEXT_PUBLIC_ADMIN_TOKEN header (AdminAuth on /workspaces)
3. Dockerfile: accept NEXT_PUBLIC_ADMIN_TOKEN as build arg
All three are required for the canvas to work in local Docker where
canvas (port 3000) fetches from platform (port 8080) cross-origin.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>