#### Brief overview of PR changes/additions
- Switching editor sections now clears the previous section's help
banner instead of leaving it over the new section
- Clicking X on the "Banner hidden. Undo" toast now just closes the
toast instead of suppressing every banner for that view
- Error/warning messages survive a section switch and are no longer
hidden by a leftover toast expiry timer
- Adds EditorBannerViewSwitchTest covering the repro and undo behaviour
#### Motivation for adding to Mudlet
Dismissing a banner then switching sections could show e.g. timer help
in the scripts section, confusing users.
#### Other info (issues closed, discussion etc)
Assisted-by: Claude:claude-fable-5
**Test case:** Open editor, Scripts, click X on the banner twice, open
Timers, back to Scripts - no stale Timers banner appears.
#### Demo
https://github.com/user-attachments/assets/2b06fc8d-f5cc-4b03-ace1-e157af28ec6d
