feat(settings): add appearance settings with dark mode toggle and translations

- Introduced new appearance settings page with a dark mode toggle feature.
- Added localization support for English, Spanish, and Portuguese in appearance settings.
- Removed the ThemeSelector component and updated PreferenceSelector accordingly.

worklenz-frontend/public/locales/en/settings/appearance.json

@@ -0,0 +1,5 @@
+{
+  "title": "Appearance",
+  "darkMode": "Dark Mode",
+  "darkModeDescription": "Switch between light and dark mode to customize your viewing experience."
+}