chore: remove unused UI components, dummy data, and theme provider

app/routes/not-found.tsx

@@ -0,0 +1,13 @@
+export function meta() {
+  return [{ title: "404 - Page Not Found" }];
+}
+
+export default function NotFound() {
+  return (
+    <div style={{ textAlign: "center", padding: "4rem" }}>
+      <h1>404</h1>
+      <p>Page not found.</p>
+      <a href="/">Go home</a>
+    </div>
+  );
+}