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/styles/landing.css

@@ -630,7 +630,7 @@
   padding: 2rem;
   border-radius: 16px;
   background: #14161d;
-  border: 1px solid #1e2130;
+  border: 1px solid #2a2f3d;
   transition: border-color 0.3s, transform 0.3s;
 }
 
@@ -748,7 +748,7 @@
   padding: 2rem;
   border-radius: 16px;
   background: #14161d;
-  border: 1px solid #1e2130;
+  border: 1px solid #2a2f3d;
   position: relative;
   counter-increment: landing-step;
   transition: border-color 0.3s, transform 0.3s;
@@ -912,7 +912,7 @@
   padding: 2rem;
   border-radius: 16px;
   background: #14161d;
-  border: 1px solid #1e2130;
+  border: 1px solid #2a2f3d;
   transition: border-color 0.3s ease, transform 0.3s ease;
 }
 
@@ -966,7 +966,7 @@
   padding: 2.5rem 2rem;
   border-radius: 16px;
   background: #14161d;
-  border: 1px solid #1e2130;
+  border: 1px solid #2a2f3d;
   transition: border-color 0.3s, transform 0.3s;
 }
 
@@ -1080,7 +1080,7 @@
 
 .landing-pricing-btn.outline {
   color: #e2e5eb;
-  border: 1px solid #1e2130;
+  border: 1px solid #2a2f3d;
   background: transparent;
 }