feat: initial project setup

- FastAPI backend (Python 3.12, uv)
- React 19 + TypeScript + Vite frontend
- Devcontainer with Node 22, Python 3.12, uv, opencode
- Basic app structure and project docs

frontend/src/main.tsx

@@ -0,0 +1,9 @@
+import { StrictMode } from "react";
+import { createRoot } from "react-dom/client";
+import App from "./App";
+
+createRoot(document.getElementById("root")!).render(
+  <StrictMode>
+    <App />
+  </StrictMode>,
+);