Add a dismiss/clear button to individual notification rows in the Activity
view that allows users to quickly remove completed or interrupted agents
from the list. The button appears on hover and clears the selected thread
using the existing clearCompletedActivity infrastructure.
Changes:
- Add clearSingleActivityThread() to clear individual notifications
- Add close button (X icon) to ActivityThreadRow, visible on hover
- Wire handleClearThread action through ActivityPrototypePage → ActivityThreadListPane
- Update ActivityThreadVirtualRow to check if thread is clearable