Quick search should default to the currently displayed month

When the user has selected a specific month to display, it's quite likely that they wish to search that month.

However this is currently very awkward to do.

It would be useful if Quick Search defaulted to the currently displayed chunk.
Failing that, the message list header bar could have its own search box.

Start by locating the Quick Search control and the code that tracks the currently displayed month. Confirm how the selected month is represented and how search defaults are currently set. Done means opening Quick Search while viewing a month uses that month automatically, with the existing search behavior preserved.

Written by the indexing model from the issue text.