Suppress "Current WiFi is not allowed" notification for scheduled backups

[aniketgit-hub101]

Description

Suppresses the error notification when a scheduled backup is skipped
because the current WiFi network is not allowed. This is intentional
behavior, not an error, so users should not receive an error
notification in this case.

Related Issue

Fixes #2231

Motivation and Context

When a user marks a WiFi network as disallowed, Vorta correctly skips
the backup. However, it was still sending an error notification saying
"Current WiFi is not allowed" — which is confusing since this is
expected behavior, not a failure.

The fix adds a check in the create_backup() method in scheduler.py
to suppress the notification when the message indicates WiFi is not
allowed.

How Has This Been Tested?

Code review confirms the notification delivery is now conditional on
the message not being WiFi-related. No functional change to backup
behavior — only the notification is suppressed.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the CONTRIBUTING guide.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.