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>

docs/REFACTORING.md

@@ -277,7 +277,7 @@ Track progress here. Mark tasks complete as they land and pass review.
 - [x] T4.7 — Confirm app.go is gone; smoke test both platforms
 
 ### Phase 5 — Hardening & docs
-- [ ] T5.1 — Surface errors from service + storage
+- [x] T5.1 — Surface errors from service + storage
 - [ ] T5.2 — Introduce `autostart.Manager` interface
 - [ ] T5.3 — Fill test gaps (folder filtering, cleanup, migration, concurrency)
 - [ ] T5.4 — Run `go test -race ./...` clean on both platforms