[Pomodoro] play a sound when work/break ends

When I'm focused working I often don't see when my work ends, same for my break. Plus the notification doesn't even send (and it wouldn't if i enable DND). It'd be nice to have a sound play when a time ends to make sure to not miss it

Start by locating the Pomodoro plugin and its timer-end handling, then inspect how the existing notification is triggered. Confirm the work and break completion paths, and verify that each plays a sound even when notifications are unavailable or DND is enabled.

Written by the indexing model from the issue text.