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/lib/utils.ts

@@ -19,6 +19,9 @@ export function formatRelativeTime(date: string | Date): string {
   return `${diffDay}d ago`;
 }
 
-export function cn(...classes: (string | boolean | undefined | null)[]): string {
-  return classes.filter(Boolean).join(" ");
+import { type ClassValue, clsx } from "clsx";
+import { twMerge } from "tailwind-merge";
+
+export function cn(...inputs: ClassValue[]) {
+  return twMerge(clsx(inputs));
 }