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/TESTS.md

@@ -1,6 +1,6 @@
 # GoSentry Test Suite
 
-All tests are located alongside source code in the `src/core/` package. Tests follow Go conventions with `*_test.go` filename patterns.
+All tests are located alongside source code in their respective packages under `src/`. Tests follow Go conventions with `*_test.go` filename patterns.
 
 ## Running Tests
 
@@ -172,12 +172,7 @@ Tests Linux autostart entry creation via XDG Desktop Entry files.
 
 ---
 
-## Future Test Coverage Gaps
+## Remaining Test Coverage Gaps
 
-Potential areas for additional tests:
-- Job group/folder filtering and persistence
-- Log cleanup (max file count and max age)
-- Settings persistence and migration
-- GUI integration tests (currently untested)
-- Concurrent job execution
-- Job history and run record storage
+- GUI integration tests — Fyne widget interaction is not yet tested
+- Job history and run record storage — on-disk run-record retrieval not covered