Add shared site header with account dropdown

Add SiteHeader and AccountDropdown components, replace
inline headers across homepage, roadmap, and docs layout.

app/docs/layout.tsx

@@ -2,6 +2,7 @@ import type { Metadata } from "next";
 import Link from "next/link";
 import { Globe, BookOpen, Menu } from "lucide-react";
 import { ThemeToggle } from "@/components/theme-toggle";
+import { AccountDropdown } from "@/components/account-dropdown";
 import { DocsSidebar } from "@/components/docs/sidebar";
 
 export const metadata: Metadata = {
@@ -38,6 +39,7 @@ export default function DocsLayout({
             </Link>
           </div>
           <ThemeToggle />
+          <AccountDropdown />
         </div>
       </header>