#4127 swamp inbox: with a filter active, sitting on the last row pages through the whole inbox in the background

Found in the verification reviews of swamp-club#4114 (PR #3474).

The interactive inbox (src/presentation/renderers/inbox_tui.tsx) loads the older page whenever the highlight is on the last row and a cursor remains. With a filter typed, the last matching row stays the last row while pages without matches append, so the inbox keeps requesting page after page until a match turns up or the inbox runs out. Each request is bounded and paging ends, so this is not a correctness bug, but it is network work the user did not ask for.

Related: when fzf orders results by score, an appended page can insert matches above the highlighted row, so the highlight moves to a different message just before a key press.

Suggested change: with a filter active, load older pages only on an explicit key press past the last row (as the retry after a failed load already does), or show a 'load older messages' row the user selects, and keep the highlight on the same message id when a page is appended.