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/package.json

@@ -31,7 +31,6 @@
     "@sentry/vite-plugin": "^5.1.1",
     "@stripe/stripe-js": "^8.7.0",
     "@tailwindcss/vite": "^4.2.1",
-    "@types/react-syntax-highlighter": "^15.5.13",
     "@xyflow/react": "^12.10.0",
     "axios": "^1.13.4",
     "class-variance-authority": "^0.7.1",
@@ -64,6 +63,7 @@
     "@types/node": "^24.10.9",
     "@types/react": "^19.2.5",
     "@types/react-dom": "^19.2.3",
+    "@types/react-syntax-highlighter": "^15.5.13",
     "@vitejs/plugin-react": "^5.1.1",
     "@vitest/coverage-v8": "^4.0.18",
     "eslint": "^9.39.1",