1. Replace serveStatic with Bun.file() to fix Content-Length: 0 bug
that caused Nginx to wait 12-30s per file.
2. Add section loader (spinner + 'Cargando...') while sections load.
3. Add AbortController to cancel previous fetch when switching menus.
4. Add credentials: 'same-origin' to ensure cookies are sent.
5. Add error handling for empty responses and HTTP errors.
Fixes: admin panel empty sections, 30s menu load, DOMException aborts.
Refs: production server tenerifeprop.es
bbe9a42691