add shared navbar on /account, /chat, /bee

Three-link nav (Chat / Account / Bee) with active-route highlighting,
plus a t4t Gateway home link on the left. Replaces the bespoke header
on the chat page; account and bee gain the same top-of-page nav.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/components/nav.tsx

@@ -0,0 +1,37 @@
+import { NavLink } from 'react-router';
+
+const ITEMS = [
+  { to: '/chat', label: 'Chat' },
+  { to: '/account', label: 'Account' },
+  { to: '/bee', label: 'Bee' },
+];
+
+export function Nav() {
+  return (
+    <nav className='border-b border-neutral-200 bg-white'>
+      <div className='mx-auto flex max-w-3xl items-center justify-between px-6 py-3'>
+        <a href='/' className='text-sm font-semibold tracking-tight'>
+          t4t Gateway
+        </a>
+        <ul className='flex gap-1'>
+          {ITEMS.map((item) => (
+            <li key={item.to}>
+              <NavLink
+                to={item.to}
+                className={({ isActive }) =>
+                  `inline-flex h-8 items-center rounded-md px-3 text-sm font-medium transition-colors ${
+                    isActive
+                      ? 'bg-neutral-900 text-white'
+                      : 'text-neutral-700 hover:bg-neutral-100'
+                  }`
+                }
+              >
+                {item.label}
+              </NavLink>
+            </li>
+          ))}
+        </ul>
+      </div>
+    </nav>
+  );
+}