Cancel pending timers when a child is closed (#6)

Co-authored-by: Harry Bayliss <harry@hjb.dev>
Co-committed-by: Harry Bayliss <harry@hjb.dev>

internal/app/app.go

@@ -837,6 +837,13 @@ func (st *uiState) OnChildStateChanged(string, IdleState) {
 	st.drawSidebar()
 }
 
+// OnChildClosed is the explicit-removal hook (close_process or the
+// terminal-corpse cleanup in reapChild). The UI already reflects
+// removals via the OnChildExited path and the children-map view, so
+// this is a no-op here — the timerManager is the consumer that
+// cares.
+func (st *uiState) OnChildClosed(string) {}
+
 // OnChildExited drops focus and shows the empty state if it was the
 // focused child.
 func (st *uiState) OnChildExited(c *Child) {