Describe the bug
Clicking a notification for deleted forum content opens a blank page instead of redirecting the user to a safe page with an explanation.

This occurs for both:

• deleted thread notifications
• deleted reply/post notifications

Expected behavior
Clicking a notification for deleted forum content should not open a blank page.

Instead, Submitty should redirect the user to a safe forum page and display a warning or informational message indicating that the selected thread or post has been deleted.

Examples:

• deleted thread notification → redirect to the forum index with a message
• deleted post/reply notification → redirect to the parent thread page or forum page with a message

To Reproduce
Steps to reproduce the behavior:

1. Create a forum thread.
2. Delete the thread, or create a reply and delete the reply.
3. Open the resulting notification from the home page or course notifications page.
4. Observe that a blank page is opened.

Configuration

• OS: macOS
• Browser: Chrome

Additional context
This issue is specifically about safely handling notification links whose target forum content has already been deleted.

This is separate from the notification text/truncation issue.