Only the Edit window dropped the Strip Silence rectangles and their scan
when Strip Silence closed. With the Mix window showing, a scan for old
values ran to the end and strip_scan_running() stayed true.
dialogs::show, which runs every frame, now drops them whenever neither
Strip Silence nor its Rename Selected Clips dialog is open.
edit.strip_silence_preview, strip_silence_scan and strip_settings move
from cmd/edit.rs to a new cmd/strip_silence.rs with its own specs(), as
AGENTS.md asks for new commands. edit.strip_silence reads its settings
through the same strip_settings.