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/App.tsx

@@ -0,0 +1,31 @@
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+import { BrowserRouter, Route, Routes } from "react-router-dom";
+import { Toaster as Sonner } from "@/components/ui/sonner";
+import { Toaster } from "@/components/ui/toaster";
+import { TooltipProvider } from "@/components/ui/tooltip";
+import { PreferencesProvider } from "@/contexts/PreferencesContext";
+import Index from "./pages/Index.tsx";
+import Preferences from "./pages/Preferences.tsx";
+import NotFound from "./pages/NotFound.tsx";
+
+const queryClient = new QueryClient();
+
+const App = () => (
+  <QueryClientProvider client={queryClient}>
+    <PreferencesProvider>
+      <TooltipProvider>
+        <Toaster />
+        <Sonner />
+        <BrowserRouter>
+          <Routes>
+            <Route path="/" element={<Index />} />
+            <Route path="/preferences" element={<Preferences />} />
+            <Route path="*" element={<NotFound />} />
+          </Routes>
+        </BrowserRouter>
+      </TooltipProvider>
+    </PreferencesProvider>
+  </QueryClientProvider>
+);
+
+export default App;