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>

src/components/Markdowns/MarkdownView.css

@@ -34,7 +34,7 @@
 .markdown-preview h4,
 .markdown-preview h5,
 .markdown-preview h6 {
-    font-family: "JetBrains Mono", ui-monospace, monospace;
+    font-family: "IBM Plex Mono", ui-monospace, monospace;
     font-weight: 600;
     color: hsl(var(--foreground));
     line-height: 1.3;
@@ -108,7 +108,7 @@
 
 /* Inline code */
 .markdown-preview :not(pre) > code {
-    font-family: "JetBrains Mono", ui-monospace, monospace;
+    font-family: "IBM Plex Mono", ui-monospace, monospace;
     font-size: 0.85em;
     background-color: hsl(var(--muted));
     color: hsl(var(--primary));
@@ -128,7 +128,7 @@
     box-shadow: inset 0 0 40px -20px hsl(var(--primary) / 0.25);
 }
 .markdown-preview pre code {
-    font-family: "JetBrains Mono", ui-monospace, monospace;
+    font-family: "IBM Plex Mono", ui-monospace, monospace;
     font-size: 0.85rem;
     background: transparent;
     border: none;
@@ -150,7 +150,7 @@
 }
 .markdown-preview th {
     background-color: hsl(var(--muted));
-    font-family: "JetBrains Mono", ui-monospace, monospace;
+    font-family: "IBM Plex Mono", ui-monospace, monospace;
     font-size: 0.8rem;
     text-transform: uppercase;
     letter-spacing: 0.04em;