docs: document VITE_API_BASE_URL and VITE_BASE_URL runtime configuration

- Add VITE_BASE_URL to frame-cms environment in docker-compose.yml (default /)
- Add inline comments explaining both CMS env vars
- Expand README with a runtime environment variables table and
  a subpath deployment example (e.g. serving CMS at /admin)

docker-compose.yml

@@ -31,7 +31,12 @@ services:
     depends_on:
       - frame-backend
     environment:
+      # URL of the backend API as seen by the browser.
+      # Override this when the API is not at the default path.
       VITE_API_BASE_URL: ${VITE_API_BASE_URL:-http://localhost:8080}
+      # Base path where the CMS is served (e.g. /admin for frame.example.com/admin).
+      # Defaults to / (served at the root).
+      VITE_BASE_URL: ${VITE_BASE_URL:-/}
     ports:
       - "${CMS_PORT:-3000}:80"
     healthcheck: