Add French (fr) translations

[niklasmarderx]

What

Add French language support. Currently the app supports English and German.

Why

French is widely spoken and France has a large developer community. The AZERTY layout is also unique and interesting for a typing trainer.

How

1. Open src/core/i18n.ts
2. The existing en and de translation objects show the pattern
3. Add a new fr: Translations object with French translations
4. Update the Language type: type Language = 'en' | 'de' | 'fr'
5. Add French to the language picker in settings
6. You don't need to translate exercise content — just the UI strings

Tip

There are ~100 translation keys. Focus on getting the structure right, and we can refine translations in follow-up PRs.