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/vite.config.ts

@@ -0,0 +1,21 @@
+import { defineConfig } from "vite";
+import react from "@vitejs/plugin-react-swc";
+import path from "path";
+import { componentTagger } from "lovable-tagger";
+
+// https://vitejs.dev/config/
+export default defineConfig(({ mode }) => ({
+  server: {
+    host: "::",
+    port: 8080,
+    hmr: {
+      overlay: false,
+    },
+  },
+  plugins: [react(), mode === "development" && componentTagger()].filter(Boolean),
+  resolve: {
+    alias: {
+      "@": path.resolve(__dirname, "./src"),
+    },
+  },
+}));