✨ Quality: Improve useTranslator robustness and test coverage

[galiprandi]

This PR improves the quality of the useTranslator hook by reducing code duplication, increasing robustness via defensive programming, and expanding test coverage.

💡 What:

• Extracted supported languages into a SUPPORTED_LANGUAGES constant.
• Added base constructor checks for Translator and LanguageDetector.
• Added 15+ new tests for edge cases and state transitions.

🎯 Why:

• The language list was duplicated in two places.
• Experimental browser APIs like Built-in AI require careful validation to avoid issues with faulty polyfills or missing methods.
• The hook had several uncovered branches and edge cases (e.g., busy states, unmounting, detection failures).

📊 Impact:

• Higher maintainability through code reuse.
• Better reliability across different browser environments.
• 100% statement and line coverage for the hook.

✅ Verification:

• pnpm test lib/hooks/useTranslator.test.ts --coverage
• pnpm build
• pnpm lint

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PR created automatically by Jules for task 1518299695485679997 started by @galiprandi

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