fix: design system v4 polish — home icon, mobile hamburger, contrast, font-label cleanup

- Home sidebar icon: always cyan, bg-accent-dim only when route is "/"
- Mobile TopBar: add left padding so hamburger isn't hidden behind logo
- Landing page: bump card border color (#1e2130 → #2a2f3d) for better contrast
- Replace all font-label references (40 occurrences, 19 files) with font-mono or font-sans
- Remove deprecated --font-label CSS variable from index.css
- Convert hardcoded hex in layout inline styles to CSS variables (light-mode ready)
- Add @types/react-syntax-highlighter for script builder types

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/scripts/PowerShellHighlighter.tsx

@@ -91,7 +91,7 @@ export function PowerShellHighlighter({ script, className }: Props) {
   }
 
   return (
-    <pre className={className ?? "font-label text-sm bg-card rounded-xl p-4 overflow-x-auto"}>
+    <pre className={className ?? "font-mono text-sm bg-card rounded-xl p-4 overflow-x-auto"}>
       <code>{parts}</code>
     </pre>
   )