feat(web-ui): widen main content container for better card grid layout

Increase max-width from 3xl to 4xl/5xl/6xl for xl screens to give the
homepage card grid and docs index more breathing room while keeping
the sidebar at 256px.

apps/web/app/layout.tsx

@@ -55,7 +55,7 @@ export default function RootLayout({
             </aside>
 
             <main className="flex-1 min-w-0">
-              <div className="px-8 py-10 max-w-3xl mx-auto">
+              <div className="px-8 py-10 max-w-4xl mx-auto lg:max-w-5xl xl:max-w-6xl">
                 {children}
               </div>
             </main>