turn the terminal/Gruvbox theme into the skeuo console

- rename the theme everywhere: id terminal -> skeuo, desktop sim skeuo -> desktop,
  skeleton terminal shell -> SkeuoLayout, page.css -> shared.css (shared components),
  old skeuo.css -> desktop.css, storage key theme -> dmx-theme (with migration)
- rebuild the theme as a walnut-and-aluminium home-lab console: brass plates, patch bay,
  analogue gym gauges, clipboard, stitched leather logbook, service rack with pilot lamps,
  framed print, chart recorder, cartridge, software box, pegboard tags, card-index drawer
- all materials are CSS + inline-SVG noise (no texture images); Barlow Semi Condensed +
  Courier Prime via next/font with fallbacks
- desktop mode is now phone-only (<=768px): switch hidden above that, stored desktop
  preference overridden, sim never mounted on wide viewports
- drop the ASCII train background and its gruvbox.webp asset

src/app/page.tsx

@@ -1,6 +1,7 @@
 import { Metadata } from 'next';
-import './page.css';
+import './shared.css';
 import './skeuo.css';
+import './desktop.css';
 import ThemeRoot from './components/ThemeRoot';
 
 export const metadata: Metadata = {