Add shared site header with account dropdown

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

components/site-header.tsx

@@ -0,0 +1,37 @@
+"use client";
+
+import Link from "next/link";
+import { Globe } from "lucide-react";
+import { ThemeToggle } from "@/components/theme-toggle";
+import { AccountDropdown } from "@/components/account-dropdown";
+
+export function SiteHeader() {
+  return (
+    <header className="border-b">
+      <div className="container mx-auto flex h-14 items-center justify-between px-4">
+        <div className="flex items-center gap-2">
+          <Globe className="size-5 text-primary" />
+          <Link href="/" className="text-lg font-semibold tracking-tight">
+            VectorDNS
+          </Link>
+        </div>
+        <div className="flex items-center gap-4">
+          <Link
+            href="/docs"
+            className="text-sm text-muted-foreground transition-colors hover:text-foreground"
+          >
+            Docs
+          </Link>
+          <Link
+            href="/roadmap"
+            className="text-sm text-muted-foreground transition-colors hover:text-foreground"
+          >
+            Roadmap
+          </Link>
+          <ThemeToggle />
+          <AccountDropdown />
+        </div>
+      </div>
+    </header>
+  );
+}