feat: add frontend source code

Add search-zen-50 React SPA source code to frontend/ directory.
Build artifacts (dist, node_modules, lock files) are gitignored.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/vitest.config.ts

@@ -0,0 +1,16 @@
+import { defineConfig } from "vitest/config";
+import react from "@vitejs/plugin-react-swc";
+import path from "path";
+
+export default defineConfig({
+  plugins: [react()],
+  test: {
+    environment: "jsdom",
+    globals: true,
+    setupFiles: ["./src/test/setup.ts"],
+    include: ["src/**/*.{test,spec}.{ts,tsx}"],
+  },
+  resolve: {
+    alias: { "@": path.resolve(__dirname, "./src") },
+  },
+});