Build in the review tool.

web/vite.config.ts

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+import { defineConfig } from 'vite';
+import react from '@vitejs/plugin-react';
+
+// Minimal typing for process.env so we don't need @types/node just for this.
+declare const process: { env: Record<string, string | undefined> };
+
+// Where a `vite dev` session sends its API calls. Playpen prefers 8420 and walks
+// upward if it's taken, so this is right unless you have two windows open — in
+// which case read the real one out of PLAYPEN_REVIEW_URL in any of its terminals.
+const apiPort = process.env.PLAYPEN_REVIEW_PORT || '8420';
+const apiTarget = `http://127.0.0.1:${apiPort}`;
+
+export default defineConfig({
+  plugins: [react()],
+
+  // Relative asset URLs, because the page is served from a tab's path
+  // (`/t/tab3/`) and not from the server root. With an absolute base the
+  // browser would ask for `/assets/app.js` and get the tab router instead.
+  base: './',
+
+  server: {
+    port: 5173,
+    proxy: {
+      // A tab's own API and event stream. Matched as a regex so the HTML at
+      // `/t/<id>/` still comes from Vite — only the calls underneath it are
+      // forwarded, which is what lets hot reloading work against a live review.
+      //
+      // Open http://localhost:5173/t/<tabId>/ to develop against a real tab.
+      '^/t/[^/]+/api': { target: apiTarget, changeOrigin: true },
+      // The app-wide endpoints: /api/tabs and /api/health.
+      '/api': { target: apiTarget, changeOrigin: true },
+    },
+  },
+
+  build: {
+    outDir: 'dist',
+    emptyOutDir: true,
+
+    // Fixed filenames rather than Vite's content hashes. The bundle is embedded
+    // in the playpen binary (src/review/assets.zig), so the Zig side has to be
+    // able to name the files at compile time — and cache busting buys nothing
+    // for a bundle that only changes when the binary does.
+    rollupOptions: {
+      output: {
+        entryFileNames: 'assets/app.js',
+        chunkFileNames: 'assets/app.js',
+        assetFileNames: 'assets/app[extname]',
+        // One file, so there is one name to embed. The app is a few hundred
+        // kilobytes loaded from localhost; splitting it saves nothing.
+        manualChunks: undefined,
+      },
+    },
+  },
+});