T5.3: Fill test gaps — cleanup, folder filtering, migration, concurrent RunDue

- src/runner/cleanup_test.go (new): 7 tests for CleanupLogs covering
  age-based deletion, count-based trimming (oldest removed, newest kept),
  non-.log files and subdirs are never touched, and zero-limit behaviour.
- src/ui/jobs_view_test.go (new): 6 tests for filteredJobIndexes,
  folderOptions, and filterValue — the folder filter logic is now verified
  without Fyne widget interaction.
- src/storage/store_test.go: two new tests — legacy pysentry.yaml migration
  and first-run default creation.
- src/app/operations_test.go: TestRunDueSkipsJobInRunningState confirms
  RunDue honours the LastState == "Running" guard and refuses a second
  concurrent run even when NextDue is in the past.
- docs/REFACTORING.md: mark T5.3 complete.
- docs/TESTS.md: retire the covered gaps; update package path.

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

docs/REFACTORING.md

@@ -279,7 +279,7 @@ Track progress here. Mark tasks complete as they land and pass review.
 ### Phase 5 — Hardening & docs
 - [x] T5.1 — Surface errors from service + storage
 - [x] T5.2 — Introduce `autostart.Manager` interface
-- [ ] T5.3 — Fill test gaps (folder filtering, cleanup, migration, concurrency)
+- [x] T5.3 — Fill test gaps (folder filtering, cleanup, migration, concurrency)
 - [ ] T5.4 — Run `go test -race ./...` clean on both platforms
 - [ ] T5.5 — Update docs (ARCHITECTURE.md, TESTS.md, README)