headscale: serve admin UI same-origin to avoid CORS, drop Caddyfile hack

headscale-admin is a SvelteKit build with its asset base path hardcoded
to /admin, so the earlier Caddyfile root override (to serve it at /)
broke asset loading. Revert it and instead document serving the UI via
an NPM Custom Location (/admin) on the same headscale.example.com
proxy host, which also avoids CORS entirely since the browser sees one
origin for both the UI and the API it calls.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

headscale_dev/compose.yaml

@@ -18,8 +18,6 @@ services:
   headscale-ui:
     image: ${HEADSCALE_UI_IMAGE:-goodieshq/headscale-admin:latest}
     restart: ${HEADSCALE_UI_RESTART:-unless-stopped}
-    volumes:
-      - ${VOL_PATH:-/data}/headscale/config/Caddyfile:/etc/caddy/Caddyfile:ro
     environment:
       - TZ=${TZ:-America/Vancouver}
       - HEADSCALE_URL=http://headscale:8080