T5.1: Surface background save/cleanup errors via ErrorOccurred event

Replace the two _ = discards in executeRun (SaveJobs + CleanupLogs after
an async run) and the _ = in RunDue (SaveJobs before a scheduled run)
with captured errors emitted as ErrorOccurred events after the state
lock is released. The UI subscriber in mainwindow.go handles the new
event by appending an "Error" record to History so failed saves are
visible to the user instead of silently dropped.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/events.go

@@ -41,9 +41,18 @@ type SchedulerStateChanged struct {
 	Paused bool
 }
 
+// ErrorOccurred signals a background error that could not be returned to a
+// caller — typically a failed save or cleanup after an async run. The UI
+// surfaces it in the History tab so the user is not silently left with
+// un-persisted state.
+type ErrorOccurred struct {
+	Err error
+}
+
 func (JobChanged) isEvent()            {}
 func (RunRecorded) isEvent()           {}
 func (SchedulerStateChanged) isEvent() {}
+func (ErrorOccurred) isEvent()         {}
 
 // Observer receives events emitted by the Service. OnEvent is the single
 // reaction point; the UI implements it and marshals any widget work onto the