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/signup/page.test.tsx

@@ -21,20 +21,20 @@ afterEach(() => cleanup());
 test("renders signup form with title", () => {
   renderWithProviders(<SignupPage />);
   // Step 1 shows "Join with Invite" title (invite code entry)
-  expect(screen.getByRole("heading", { name: "Join with Invite" })).toBeDefined();
+  expect(screen.getByRole("heading", { name: "Únete con Invitación" })).toBeDefined();
 });
 
 test("renders invite code input", () => {
   renderWithProviders(<SignupPage />);
-  expect(screen.getByLabelText("Invite Code")).toBeDefined();
+  expect(screen.getByLabelText("Código de Invitación")).toBeDefined();
 });
 
 test("renders continue button", () => {
   renderWithProviders(<SignupPage />);
-  expect(screen.getByRole("button", { name: "Continue" })).toBeDefined();
+  expect(screen.getByRole("button", { name: "Continuar" })).toBeDefined();
 });
 
 test("renders link to login", () => {
   renderWithProviders(<SignupPage />);
-  expect(screen.getByText("Sign in")).toBeDefined();
+  expect(screen.getByText("Iniciar sesión")).toBeDefined();
 });