feat(ui): public marketing landing page at "/" for logged-out visitors

Add a LandingPage (hero + features + how-it-works + CTAs) served at "/" so
visitors learn about the product before signing in — matching the JobTrack
mockups (indigo/cyan, dark hero + light sections). If the visitor already has a
session, LandingPage redirects into the app (/jobs); otherwise it shows the
marketing page with "Sign in" CTAs. The "/" route is public (outside the
auth-gated Shell), so logged-out users no longer bounce straight to /login.

Verified live: renders at "/" with headline, feature grid, how-it-works steps
and CTAs; no console errors; type-clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

job-tracker-ui/src/App.tsx

@@ -28,6 +28,7 @@ import JobTable from "./components/JobTable";
 import type { JobTableColumns } from "./components/JobTable";
 import { I18nProvider, useI18n } from "./i18n/I18nProvider";
 import LoginPage from "./pages/LoginPage";
+import LandingPage from "./pages/LandingPage";
 import ForgotPasswordPage from "./pages/ForgotPasswordPage";
 import ResetPasswordPage from "./pages/ResetPasswordPage";
 import RouteErrorPage from "./pages/RouteErrorPage";
@@ -344,6 +345,7 @@ export default function App() {
   });
 
   const router = useMemo(() => createBrowserRouter([
+    { path: "/", element: <LandingPage />, errorElement: <RouteErrorPage /> },
     { path: "/login", element: <LoginPage />, errorElement: <RouteErrorPage /> },
     { path: "/forgot-password", element: <ForgotPasswordPage />, errorElement: <RouteErrorPage /> },
     { path: "/reset-password", element: <ResetPasswordPage />, errorElement: <RouteErrorPage /> },