Fix login button visibility and Enter key submission
- Create spectating store for cross-component state sharing
- Show header login button when spectating on homepage
- Remove "Want to play?" banner (replaced by header login button)
- Add explicit Enter key handler to main login form input
Fixes:
- Login button now shows when spectating (games view)
- No login button on home login screen (avoids duplicate UI)
- Enter key explicitly triggers login on main form
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>