**Motivations:**
- Allow users to set their preferred language (fr/en) in the settings page
- Load language preference from localStorage at startup to configure the application locale
**Root causes:**
- Language preference was only available in the header via LanguageSelector component
- Language preference was stored in IndexedDB instead of localStorage
- No centralized language settings management in the settings page
**Correctifs:**
- Created LanguageSettingsManager component for settings page
- Migrated language storage from IndexedDB to localStorage for consistency
- Updated _app.tsx to load locale from localStorage synchronously at startup
- Updated useI18n hook to use localStorage instead of IndexedDB
- Updated LanguageSelector component to use localStorage instead of IndexedDB
**Evolutions:**
- Added language preference section in settings page (displayed first)
- Language preference is now loaded at application startup from localStorage
- Added translations for language settings (settings.language.*)
**Pages affectées:**
- components/LanguageSettingsManager.tsx (new)
- pages/settings.tsx
- pages/_app.tsx
- hooks/useI18n.ts
- components/LanguageSelector.tsx
- locales/fr.txt
- locales/en.txt
