T3.3: Add state-mutating operations to app.Service

Add src/app/operations.go with the seven intents that make the Service
the sole writer of job and runtime state: CreateJob, UpdateJob,
DeleteJob, SetEnabled, RunNow, SetGlobalPause, UpdateSettings. Each
returns error, persists through the store, and announces changes via
RunRecorded/JobChanged/SchedulerStateChanged events.

Extend the Service with a parsed-schedule cache, a global paused flag,
an injectable runJob seam (defaults to runner.RunJob) for testing the
run-now path, and a lifecycle ctx. Run and next-run timing now live in
the Service (duplicating the scheduler temporarily); T3.4 converts the
scheduler to drive the Service and removes the duplication.

Autostart is left to the caller until T5.2's injectable Manager; async
save errors in the run goroutine remain deferred to T5.1. Adds 12 tests
covering create/update/delete, enable/pause, global pause, run-now with
a fake runner, and settings persistence/validation.

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

docs/REFACTORING.md

@@ -262,7 +262,7 @@ Track progress here. Mark tasks complete as they land and pass review.
 ### Phase 3 — Application service layer
 - [x] T3.1 — Create `src/app/service.go`; owns state behind mutex
 - [x] T3.2 — Add `src/app/events.go`; Event types + Observer
-- [ ] T3.3 — Add state-mutating operations to service
+- [x] T3.3 — Add state-mutating operations to service
 - [ ] T3.4 — Convert `scheduler` to use service; inject Clock
 - [ ] T3.5 — Move display helpers to `src/app/format.go`
 - [ ] T3.6 — Add `src/app` unit tests (no Fyne)