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/components/WindowsDesktop.tsx

@@ -577,10 +577,10 @@ export default function WindowsDesktop() {
             type="button"
             className="win-theme-toggle"
             onClick={toggle}
-            title={`Switch to ${theme === "skeuo" ? "gruvbox" : "Desktop"} theme`}
+            title={`Switch to ${theme === "desktop" ? "skeuo" : "Desktop"} theme`}
           >
-            <span aria-hidden="true">{theme === "skeuo" ? "▚" : "❖"}</span>
-            <span>{theme === "skeuo" ? "Gruvbox" : "Desktop"}</span>
+            <span aria-hidden="true">{theme === "desktop" ? "▚" : "❖"}</span>
+            <span>{theme === "desktop" ? "Skeuo" : "Desktop"}</span>
           </button>
 
           <Clock className="win-clock" />