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

package-lock.json

@@ -25,6 +25,7 @@
         "@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",
@@ -34,6 +35,7 @@
         "react-dom": "^19.0.0",
         "shiki": "^3.22.0",
         "stripe": "^20.3.1",
+        "tailwind-merge": "^3.4.0",
         "zod": "^4.3.6"
       },
       "devDependencies": {
@@ -2395,6 +2397,15 @@
       "integrity": "sha512-IV3Ou0jSMzZrd3pZ48nLkT9DA7Ag1pnPzaiQhpW7c3RbcqqzvzzVu+L8gfqMp/8IM2MQtSiqaCxrrcfu8I8rMA==",
       "license": "MIT"
     },
+    "node_modules/clsx": {
+      "version": "2.1.1",
+      "resolved": "https://registry.npmjs.org/clsx/-/clsx-2.1.1.tgz",
+      "integrity": "sha512-eYm0QWBtUrBWZWG0d386OGAw16Z995PiOVo2B7bjWSbHedGl5e0ZWaq65kOGgUSNesEIDkB9ISbTg/JK9dhCZA==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=6"
+      }
+    },
     "node_modules/cluster-key-slot": {
       "version": "1.1.2",
       "resolved": "https://registry.npmjs.org/cluster-key-slot/-/cluster-key-slot-1.1.2.tgz",
@@ -4156,6 +4167,16 @@
         "node": ">=16 || 14 >=14.17"
       }
     },
+    "node_modules/tailwind-merge": {
+      "version": "3.4.0",
+      "resolved": "https://registry.npmjs.org/tailwind-merge/-/tailwind-merge-3.4.0.tgz",
+      "integrity": "sha512-uSaO4gnW+b3Y2aWoWfFpX62vn2sR3skfhbjsEnaBI81WD1wBLlHZe5sWf0AqjksNdYTbGBEd0UasQMT3SNV15g==",
+      "license": "MIT",
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/sponsors/dcastil"
+      }
+    },
     "node_modules/tailwindcss": {
       "version": "4.1.18",
       "resolved": "https://registry.npmjs.org/tailwindcss/-/tailwindcss-4.1.18.tgz",