Redraw interface after launching external viewer

GUI programs tend to output warnings on their stdout or stderr.
In paperboy, those warnings overwrite part of the user interface:

It should be sufficient to redraw the interface when the external viewer terminates, as paperboy is unresponsive while waiting for the external program anyway.

Look for the code that launches the external viewer and waits for it to terminate, likely in a module handling external processes or the main UI loop. After the viewer process ends, trigger a redraw of the TUI interface. Check existing tests for UI redrawing or process handling to understand the pattern.

索引モデルが issue の本文から書いたものです。