feat: add shared design tokens, JetBrains Mono font, and fix cn() utility

- Add CSS custom properties for surfaces, text, borders, accent, radius, font stacks
- Add JetBrains Mono via next/font/google alongside Inter (both as CSS variables)
- Upgrade cn() from naive filter/join to twMerge(clsx()) for proper Tailwind class merging
- Standardize marketing section containers from max-w-7xl to max-w-6xl
- Install tailwind-merge and clsx dependencies

apps/web/package.json

@@ -16,6 +16,7 @@
     "@dagrejs/dagre": "^2.0.4",
     "@xyflow/react": "^12.10.0",
     "bcryptjs": "^3.0.3",
+    "clsx": "^2.1.1",
     "ioredis": "^5.9.2",
     "lucide-react": "^0.469.0",
     "next": "^15.1.0",
@@ -25,6 +26,7 @@
     "react-dom": "^19.0.0",
     "shiki": "^3.22.0",
     "stripe": "^20.3.1",
+    "tailwind-merge": "^3.4.0",
     "zod": "^4.3.6"
   },
   "devDependencies": {