Add "Negate" option to the "Downtime comment (regex)" view filter

Currently, the "Downtime comment (regex)" filter in custom views does not provide a Negate option, unlike several other view filters in Checkmk.

This makes it impossible to create views such as:

Show all scheduled downtimes except those with specific comments.

For example, I would like to exclude automatically created downtimes with comments like:

-Testing
-Check
-Development
-Veeam Backups - tägliche Sicherung

Because the view filter uses POSIX regular expressions, negative lookaheads are not supported. As a result, there is currently no way to express this requirement using the existing filter.

Adding a simple Negate checkbox to the Downtime comment (regex) filter would:

-Make the filter behavior consistent with other view filters that already support negation.
-Eliminate the need for complex or impossible regular expressions.
-Enable administrators to easily hide maintenance windows or other automatically created downtimes while still displaying all relevant manual downtimes.

This would be a small UI enhancement with a significant improvement in usability and consistency.