Add API Reference section to portal app

Introduces a new API Reference page at /reference, including a sidebar link in the header. Implements ModernApiReference with endpoint grouping, detailed documentation, and code examples. Adds supporting page and metadata for the new section.

Author: 0xFirekeeper

apps/portal/src/app/Header.tsx

@@ -27,6 +27,10 @@ import {
 import { ThirdwebIcon } from "../icons/thirdweb";
 
 const links = [
+  {
+    href: "/reference",
+    name: "API Reference",
+  },
   {
     href: "/wallets",
     name: "Wallets",
@@ -281,7 +285,7 @@ export function Header() {
                   }}
                 >
                   <NavLink href={link.href} name={link.name} />
-                  {pathname.startsWith(link.href) && (
+                  {pathname?.startsWith(link.href) && (
                     <div className="bg-violet-700 h-[2px] inset-x-0 rounded-full absolute -bottom-1" />
                   )}
                 </li>
@@ -526,7 +530,7 @@ function NavLink(props: {
     <Link
       className={clsx(
         "font-medium text-base transition-colors hover:text-foreground xl:text-sm",
-        pathname.startsWith(props.href)
+        pathname?.startsWith(props.href)
           ? "text-foreground"
           : "text-muted-foreground",
         props.icon ? "flex flex-row gap-3" : "",