(SP: 1) [Frontend] Guest warning before quiz start

[LesiaUKR]

Goal

Warn unauthenticated users before starting a quiz that their results won't be saved to the leaderboard, and provide easy access to login/signup.

Scope

• Show warning block (amber styling) with UserRound icon on quiz rules screen for guests
• Three buttons: Login, Sign Up (redirect with returnTo), Continue as Guest
• Logged-in users see the standard Start button unchanged
• Reuse existing quiz.result.loginButton and quiz.result.signupButton i18n keys
• Add quiz.rules.guestWarning and quiz.rules.continueAsGuest keys (uk/en/pl)

Technical steps:

1. Add i18n keys to messages/en.json, uk.json, pl.json
2. Update QuizContainer rules screen with conditional rendering based on isGuest
3. Use Link from @/i18n/routing for login/signup navigation

Expected impact

• UX: guests make an informed choice before starting
• Conversion: login/signup buttons increase registration rate

Out of scope

• Blocking guests from taking quizzes entirely
• Changing quiz result screen guest flow