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/signup/page.test.tsx

@@ -1,6 +1,7 @@
-import { render, screen, cleanup } from "@testing-library/react";
+import { screen, cleanup } from "@testing-library/react";
 import { expect, test, vi, beforeEach, afterEach } from "vitest";
 import SignupPage from "./page";
+import { renderWithProviders } from "../test-utils";
 
 const mockPush = vi.fn();
 vi.mock("next/navigation", () => ({
@@ -8,30 +9,32 @@ vi.mock("next/navigation", () => ({
   useSearchParams: () => ({ get: () => null }),
 }));
 
+const mockRegister = vi.fn();
 vi.mock("../auth-context", () => ({
-  useAuth: () => ({ user: null, register: vi.fn() }),
+  useAuth: () => ({ user: null, register: mockRegister }),
+  AuthProvider: ({ children }: { children: React.ReactNode }) => <>{children}</>,
 }));
 
 beforeEach(() => vi.clearAllMocks());
 afterEach(() => cleanup());
 
 test("renders signup form with title", () => {
-  render(<SignupPage />);
+  renderWithProviders(<SignupPage />);
   // Step 1 shows "Join with Invite" title (invite code entry)
   expect(screen.getByRole("heading", { name: "Join with Invite" })).toBeDefined();
 });
 
 test("renders invite code input", () => {
-  render(<SignupPage />);
+  renderWithProviders(<SignupPage />);
   expect(screen.getByLabelText("Invite Code")).toBeDefined();
 });
 
 test("renders continue button", () => {
-  render(<SignupPage />);
+  renderWithProviders(<SignupPage />);
   expect(screen.getByRole("button", { name: "Continue" })).toBeDefined();
 });
 
 test("renders link to login", () => {
-  render(<SignupPage />);
+  renderWithProviders(<SignupPage />);
   expect(screen.getByText("Sign in")).toBeDefined();
 });