feat: lab-terminal theme + new logo

Adopt the Hangman Lab visual language (adapted from the Gitea
STYLE.md, Gitea-only mechanics excluded):

- New acid SVG mark as favicon (?v cache-bust) and navbar logo.
- Single acid accent (#d8ff3e); retune globals.css tokens to the
  ink/panel/line/text/dim/danger palette; drop the cyan/violet pair
  (secondary -> same acid so existing components stay on-brand).
- Fonts: IBM Plex Mono for body/UI/mono, Major Mono Display for the
  lowercase brand lockup; Tailwind fontFamily + content fonts updated.
- Site-wide fixed 48px blueprint-grid backdrop + one acid sheen.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

public/index.html

@@ -5,11 +5,11 @@
         <meta name="viewport" content="width=device-width, initial-scale=1.0">
         <meta name="color-scheme" content="dark">
         <title>Hangman Lab</title>
-        <link rel="icon" type="image/png" href="./icons/logo.png">
+        <link rel="icon" type="image/svg+xml" href="./icons/hangman-lab-logo.svg?v=hl-20260517">
         <link rel="preconnect" href="https://fonts.googleapis.com">
         <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
         <link
-            href="https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700&family=JetBrains+Mono:wght@400;500;600;700&display=swap"
+            href="https://fonts.googleapis.com/css2?family=Major+Mono+Display&family=IBM+Plex+Mono:ital,wght@0,400;0,500;0,600;1,400&display=swap"
             rel="stylesheet"
         />
     </head>