Add user notification preferences and related API endpoints

- Introduced a new model `UserNotificationPreferences` to store user-specific notification dismissal settings.
- Added context processor to include notification preferences in templates.
- Implemented API endpoints to dismiss backup and AI scanner notifications permanently.
- Updated front-end logic to handle notification dismissal via server-side checks and API calls.
- Enhanced documentation to include a new Custom CSS Guide for theme customization.

baseTemplate/apps.py

@@ -6,3 +6,6 @@ from django.apps import AppConfig
 
 class BasetemplateConfig(AppConfig):
     name = 'baseTemplate'
+    
+    def ready(self):
+        import baseTemplate.signals