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/sidebar/ActivityItem.tsx

@@ -91,12 +91,12 @@ export function ActivityItem({
 
       {/* Ticket number or timestamp */}
       {ticketNumber && !isRecent && (
-        <span className="shrink-0 font-label text-[0.5625rem] text-[#60a5fa]">
+        <span className="shrink-0 font-mono text-[0.5625rem] text-[#60a5fa]">
           {ticketNumber}
         </span>
       )}
       {isRecent && timestamp && (
-        <span className="shrink-0 font-label text-[0.5625rem] text-text-muted">
+        <span className="shrink-0 font-mono text-[0.5625rem] text-text-muted">
           {formatRelativeTime(timestamp)}
         </span>
       )}