Phase 5: Translate Auth Pages - login and signup

- Create auth.json translation files for es, en, ca
- Translate login page: title, subtitle, form labels, buttons, footer
- Translate signup page: invite code step and account creation step
- Translate signup/[code] redirect page
- Update IntlProvider to load auth namespace
- Update test expectations to match Spanish translations (default language)
- All frontend and e2e tests passing

frontend/app/login/page.test.tsx

@@ -19,21 +19,21 @@ afterEach(() => cleanup());
 
 test("renders login form with title", () => {
   renderWithProviders(<LoginPage />);
-  expect(screen.getByText("Welcome back")).toBeDefined();
+  expect(screen.getByText("Bienvenido de nuevo")).toBeDefined();
 });
 
 test("renders email and password inputs", () => {
   renderWithProviders(<LoginPage />);
-  expect(screen.getByLabelText("Email")).toBeDefined();
-  expect(screen.getByLabelText("Password")).toBeDefined();
+  expect(screen.getByLabelText("Correo electrónico")).toBeDefined();
+  expect(screen.getByLabelText("Contraseña")).toBeDefined();
 });
 
 test("renders sign in button", () => {
   renderWithProviders(<LoginPage />);
-  expect(screen.getByRole("button", { name: "Sign in" })).toBeDefined();
+  expect(screen.getByRole("button", { name: "Iniciar sesión" })).toBeDefined();
 });
 
 test("renders link to signup", () => {
   renderWithProviders(<LoginPage />);
-  expect(screen.getByText("Sign up")).toBeDefined();
+  expect(screen.getByText("Regístrate")).toBeDefined();
 });