T3.6: Add src/app unit tests (no Fyne)

Fill the test gaps in the app package, raising coverage from 82% to 97%:

- format_test.go: cover all display helpers moved in T3.5 (StatusText,
  EventText, Display*), previously untested.
- operations_test.go: add RunNow not-found and refused-while-running
  paths, DeleteJob/SetEnabled not-found errors, UpdateJob re-enable,
  runtimeForLocked lazy-recreate, UpdateSettings invalid-config branches,
  and prependLog activity-list capping.

All tests are Fyne-free, using a temp-dir store, a fake runner seam, and
a fake Clock. go vet and go test -race pass across the module.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/REFACTORING.md

@@ -265,7 +265,7 @@ Track progress here. Mark tasks complete as they land and pass review.
 - [x] T3.3 — Add state-mutating operations to service
 - [x] T3.4 — Convert `scheduler` to use service; inject Clock
 - [x] T3.5 — Move display helpers to `src/app/format.go`
-- [ ] T3.6 — Add `src/app` unit tests (no Fyne)
+- [x] T3.6 — Add `src/app` unit tests (no Fyne)
 
 ### Phase 4 — Carve up the GUI
 - [ ] T4.1 — Rename `gui` → `ui`; split app.go into run.go + mainwindow.go