- Add language and theme selector submenus to UserMenu component for quick access
- Refactor shared utilities: extract THEME_OPTIONS constant and getLocaleDisplayName() function
- Update LocaleSelect and ThemeSelect to use shared utilities, eliminating code duplication
- Make UserMenu reactive with MobX observer for real-time setting updates
- Fix language switching reactivity by immediately updating workspaceStore.state.locale
- Add scrollable menu support for language selector (max-h-[90vh])
- Apply same instant locale update to PreferencesSection for consistency

🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>