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/guides/GuideCard.tsx

@@ -24,7 +24,7 @@ export function GuideCard({ guide }: GuideCardProps) {
           <p className="text-xs text-muted-foreground leading-relaxed">
             {guide.summary}
           </p>
-          <span className="mt-2 inline-block font-label text-[0.625rem] uppercase tracking-widest text-primary">
+          <span className="mt-2 inline-block font-sans text-[0.625rem] uppercase tracking-widest text-primary">
             {guide.sections.length} {guide.sections.length === 1 ? 'section' : 'sections'}
           </span>
         </div>

frontend/src/components/guides/GuideSection.tsx

@@ -18,7 +18,7 @@ export function GuideSection({ section, index }: GuideSectionProps) {
       <ol className="space-y-3 pl-8">
         {section.steps.map((step, i) => (
           <li key={i} className="relative">
-            <span className="absolute -left-6 top-0.5 font-label text-[0.625rem] text-muted-foreground">
+            <span className="absolute -left-6 top-0.5 font-sans text-[0.625rem] text-muted-foreground">
               {i + 1}.
             </span>
             <p