Add React UI with Vite

Dashboard, agents, goals, issues, and projects pages with sidebar
navigation. API client layer, custom hooks, and shared layout
components. Built with Vite and TypeScript.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/pages/Dashboard.tsx

@@ -0,0 +1,8 @@
+export function Dashboard() {
+  return (
+    <div>
+      <h2 className="text-2xl font-bold mb-4">Dashboard</h2>
+      <p className="text-gray-600">Welcome to Paperclip. Select a section from the sidebar.</p>
+    </div>
+  );
+}