fix: loading screen via Go handler instead of file:// HTML

- bee-web.service: remove After=bee-audit so Go starts immediately
- Go serves loading page from / when audit JSON not yet present;
  JS polls /api/ready (503 until file exists, 200 when ready)
  then redirects to dashboard
- bee-openbox-session: wait for /healthz (Go binds fast <2s),
  open http://localhost/ directly — no file:// cross-origin issues
- Remove loading.html static file

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

iso/overlay/etc/systemd/system/bee-web.service

@@ -1,6 +1,6 @@
 [Unit]
 Description=Bee: hardware audit web viewer
-After=bee-network.service bee-audit.service
+After=bee-network.service
 Wants=bee-audit.service
 
 [Service]