Underground FX WOW Rebuild

This pack pushes the site harder around the supplied Underground FX image.

Included in this pass:
- Underground-image-first background shell
- stronger glow / sparkle / particle / glass motion system
- refreshed flagship homepage
- refreshed admin control-room shell
- Bootstrap 5 + Bootstrap Icons in the master shell
- now playing polling via api/nowplaying.php
- all existing public feature routes preserved in the nav
- HomeTime-style feature stack kept as the functional source layer

Main files changed:
- assets/css/ugfx-master.css
- assets/js/ugfx-master.js
- assets/images/ugfx-underground-stage.png
- includes/public_bootstrap.php
- index.php
- admin/includes/admin_bootstrap.php
- admin/index.php