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/src/app/globals.css

@@ -1 +1,38 @@
 @import "tailwindcss";
+
+@layer base {
+  :root {
+    /* Surfaces */
+    --surface-page: #0a0a0a;
+    --surface-card: rgb(23 23 23); /* neutral-900 */
+    --surface-card-hover: rgb(38 38 38 / 0.5); /* neutral-800/50 */
+    --surface-elevated: rgb(23 23 23); /* neutral-900 */
+    --surface-input: rgb(10 10 10); /* neutral-950 */
+
+    /* Text */
+    --text-primary: rgb(245 245 245); /* neutral-100 */
+    --text-secondary: rgb(163 163 163); /* neutral-400 */
+    --text-muted: rgb(115 115 115); /* neutral-500 */
+
+    /* Borders */
+    --border-default: rgb(38 38 38); /* neutral-800 */
+    --border-subtle: rgb(38 38 38 / 0.5); /* neutral-800/50 */
+    --border-strong: rgb(64 64 64); /* neutral-700 */
+
+    /* Accent (AgentLens emerald) */
+    --accent: #10b981;
+    --accent-hover: #34d399;
+    --accent-muted: rgba(16, 185, 129, 0.15);
+    --accent-foreground: #0a0a0a;
+
+    /* Radius */
+    --radius-card: 1rem;
+    --radius-button: 0.5rem;
+    --radius-icon: 0.75rem;
+    --radius-badge: 9999px;
+
+    /* Fonts */
+    --font-sans: var(--font-inter), system-ui, sans-serif;
+    --font-mono: var(--font-jetbrains), 'JetBrains Mono', 'Fira Code', monospace;
+  }
+}