Add AuthGateStep page (wizard step 5)

- Centered-form layout with 3 progressive sub-steps: SSO/email → details → verify
- Google/Microsoft SSO buttons + email entry (sub-step 1)
- Name, phone, contact preference fields (sub-step 2) — phone optional when email-only
- 6-digit verification code entry (sub-step 3)
- Benefit framing ("Save your plan") not gate framing
- Responsive name fields (stacked on mobile, side-by-side on desktop)
- autoComplete + inputMode on all fields per WCAG 3.3.8
- Audit: 18/20 (Excellent), P1 fixed (responsive name fields)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/pages/AuthGateStep/index.ts

@@ -0,0 +1,8 @@
+export { AuthGateStep, default } from './AuthGateStep';
+export type {
+  AuthGateStepProps,
+  AuthGateStepValues,
+  AuthGateStepErrors,
+  AuthSubStep,
+  ContactPreference,
+} from './AuthGateStep';