feat: add React app shell with routing

src/pages/ChatPage.tsx

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+export default function ChatPage() {
+  return <div>Chat</div>;
+}

src/pages/LoginPage.tsx

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+export default function LoginPage() {
+  return <div>Login</div>;
+}

src/pages/WorkspacePage.tsx

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+export default function WorkspacePage() {
+  return <div>Workspaces</div>;
+}