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/sidebar.json

@@ -10,5 +10,6 @@
   "team-members": "Team Members",
   "teams": "Teams",
   "change-password": "Change Password",
-  "language-and-region": "Language and Region"
+  "language-and-region": "Language and Region",
+  "appearance": "Appearance"
 }