Phase 2: Language Context & Selector - Add language dropdown to Header and auth pages

- Create LanguageSelector component with dropdown (shows flag + name)
- Add LanguageSelector to Header (right side, near user email/logout)
- Add LanguageSelector to login, signup, and signup/[code] pages
- Create test-utils.tsx with renderWithProviders helper
- Add vitest.setup.ts to mock localStorage
- Update all test files to use renderWithProviders
- Language selector persists choice in localStorage
- HTML lang attribute updates dynamically based on selected language

All frontend and e2e tests passing.

frontend/app/test-utils.tsx

@@ -0,0 +1,16 @@
+import { ReactElement } from "react";
+import { render, RenderOptions } from "@testing-library/react";
+import { Providers } from "./components/Providers";
+import { AuthProvider } from "./auth-context";
+
+function AllProviders({ children }: { children: React.ReactNode }) {
+  return (
+    <Providers>
+      <AuthProvider>{children}</AuthProvider>
+    </Providers>
+  );
+}
+
+export function renderWithProviders(ui: ReactElement, options?: Omit<RenderOptions, "wrapper">) {
+  return render(ui, { wrapper: AllProviders, ...options });
+}