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/layout/QuickLaunch.tsx

@@ -144,11 +144,11 @@ export function QuickLaunch({ open, onClose }: QuickLaunchProps) {
 
         <div className="flex items-center gap-4 border-t border-border px-4 py-2">
           <span className="flex items-center gap-1 text-[0.625rem] text-muted-foreground">
-            <kbd className="rounded border border-border bg-background px-1 py-px font-label">↑↓</kbd>
+            <kbd className="rounded border border-border bg-background px-1 py-px font-mono">↑↓</kbd>
             Navigate
           </span>
           <span className="flex items-center gap-1 text-[0.625rem] text-muted-foreground">
-            <kbd className="rounded border border-border bg-background px-1 py-px font-label">↵</kbd>
+            <kbd className="rounded border border-border bg-background px-1 py-px font-mono">↵</kbd>
             Open
           </span>
         </div>