frontend: add mobile auth page visual regression coverage

## Problem Auth pages are key first-impression screens and need mobile coverage for button visibility and layout. ## Scope - Add or extend Playwright screenshot tests for `/login` and `/register` at mobile width. - Cover the new cross-navigation buttons. ## Acceptance criteria - Buttons are visible without overlap on mobile. - Screenshot baseline or assertion is documented. ## Validation Run: `npm run frontend:test:e2e -- --grep auth`