chore(app): scaffold tauri 2 + react + vite, window launches

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/main.tsx

@@ -0,0 +1,10 @@
+import React from "react";
+import ReactDOM from "react-dom/client";
+
+ReactDOM.createRoot(document.getElementById("root")!).render(
+  <React.StrictMode>
+    <div style={{ color: "#ddd", fontFamily: "monospace", padding: 16 }}>
+      spacesh — scaffold OK
+    </div>
+  </React.StrictMode>
+);