feat: scaffold web frontend

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

web/src/main.tsx

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+import { StrictMode } from "react";
+import { createRoot } from "react-dom/client";
+import "@/styles/index.css";
+
+createRoot(document.getElementById("root")!).render(
+  <StrictMode>
+    <div className="min-h-screen bg-neutral-950 p-8 text-neutral-100">
+      <h1 className="text-2xl font-semibold">Codespace</h1>
+      <p className="mt-2 text-sm text-neutral-400">
+        Web frontend scaffold. IDE shell coming in later tasks.
+      </p>
+    </div>
+  </StrictMode>,
+);

web/src/styles/index.css

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+@import "tailwindcss";

web/src/vite-env.d.ts

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+ /// <reference types="vite/client" />