Allow hiding the overview row in the Dopesheet view

It may be useful to add an option to the Dopesheet filters to hide the overview row.

Currently, when the `Offset` button is enabled, selecting a key on the overview row allows all corresponding visible timeline keys to be offset together. This is useful behavior, especially when working with long animations.

However, when box selecting a large number of keys, it can be easy to accidentally include keys on the overview row. Since including an overview key changes which keys are affected by the operation, users who only want to edit specific timeline keys need to be more careful with their selection.

This can be particularly inconvenient for workflows where the animator wants to keep many different timelines visible at once in order to compare their timing, such as checking how many frames apart rotate and translate keys are. In these cases, hiding timeline types with the existing filters is not always a suitable workaround.

Adding a filter option to hide the overview row would provide a safer selection workflow without reverting the current behavior of overview keys.

Related forum thread:
https://esotericsoftware.com/forum/d/30627-%E5%85%B3%E4%BA%8E%E6%91%84%E5%BD%B1%E8%A1%A8%E4%B8%AD%E7%9A%84%E5%81%8F%E7%A7%BB%E9%80%89%E9%A1%B9/10

Start by locating the Dopesheet filter controls and the overview-row rendering and selection paths. Check how existing timeline-type filters are wired and how Offset affects selection. Done means a filter option hides only the overview row while preserving current overview-key behavior and the other Dopesheet filters.

Written by the indexing model from the issue text.