- Extract preference logic into dedicated hooks (useUserLocale, useUserTheme)
- Add applyLocaleEarly() for consistent early application
- Remove applyUserPreferences() from user store (now redundant)
- Simplify App.tsx by moving effects to custom hooks
- Make locale/theme handling consistent and reactive
- Clean up manual preference calls from sign-in flows

Fixes locale not overriding localStorage on user login.
Improves maintainability with better separation of concerns.