Restructure frontend code to allow for extensions (#4721)

# Description of Changes
Move frontend code into `core` folder and add infrastructure for
`proprietary` folder to include premium, non-OSS features

frontend/src/proprietary/App.tsx

@@ -0,0 +1,37 @@
+import { Suspense } from "react";
+import { Routes, Route } from "react-router-dom";
+import { AppProviders } from "@app/components/AppProviders";
+import { LoadingFallback } from "@app/components/shared/LoadingFallback";
+import Landing from "@app/routes/Landing";
+import Login from "@app/routes/Login";
+import Signup from "@app/routes/Signup";
+import AuthCallback from "@app/routes/AuthCallback";
+import OnboardingTour from "@app/components/onboarding/OnboardingTour";
+
+// Import global styles
+import "@app/styles/tailwind.css";
+import "@app/styles/cookieconsent.css";
+import "@app/styles/index.css";
+import "@app/styles/auth-theme.css";
+
+// Import file ID debugging helpers (development only)
+import "@app/utils/fileIdSafety";
+
+export default function App() {
+  return (
+    <Suspense fallback={<LoadingFallback />}>
+      <AppProviders>
+        <Routes>
+          {/* Auth routes - no nested providers needed */}
+          <Route path="/login" element={<Login />} />
+          <Route path="/signup" element={<Signup />} />
+          <Route path="/auth/callback" element={<AuthCallback />} />
+
+          {/* Main app routes - Landing handles auth logic */}
+          <Route path="/*" element={<Landing />} />
+        </Routes>
+        <OnboardingTour />
+      </AppProviders>
+    </Suspense>
+  );
+}