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>

tailwind.config.js

@@ -42,15 +42,10 @@ module.exports = {
                 sm: "calc(var(--radius) - 4px)",
             },
             fontFamily: {
-                sans: ["Inter", "ui-sans-serif", "system-ui", "sans-serif"],
-                mono: [
-                    "JetBrains Mono",
-                    "Fira Code",
-                    "ui-monospace",
-                    "SFMono-Regular",
-                    "Menlo",
-                    "monospace",
-                ],
+                // Lab-terminal: IBM Plex Mono is the body/UI/mono face.
+                sans: ["IBM Plex Mono", "ui-monospace", "SFMono-Regular", "monospace"],
+                mono: ["IBM Plex Mono", "ui-monospace", "SFMono-Regular", "monospace"],
+                display: ["Major Mono Display", "IBM Plex Mono", "monospace"],
             },
             boxShadow: {
                 glow: "0 0 0 1px hsl(var(--primary) / 0.25), 0 0 24px -6px hsl(var(--primary) / 0.45)",