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/src/test/example.test.ts

@@ -0,0 +1,7 @@
+import { describe, it, expect } from "vitest";
+
+describe("example", () => {
+  it("should pass", () => {
+    expect(true).toBe(true);
+  });
+});

frontend/src/test/setup.ts

@@ -0,0 +1,15 @@
+import "@testing-library/jest-dom";
+
+Object.defineProperty(window, "matchMedia", {
+  writable: true,
+  value: (query: string) => ({
+    matches: false,
+    media: query,
+    onchange: null,
+    addListener: () => {},
+    removeListener: () => {},
+    addEventListener: () => {},
+    removeEventListener: () => {},
+    dispatchEvent: () => {},
+  }),
+});