T4.3: Extract job_dialog.go; validate schedule via domain.Validate

Moves showJobDialog out of jobs_view.go into its own file and adds a
domain.Validate call on the schedule field so malformed cron expressions
are rejected with an error dialog before onSave is invoked.

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

docs/REFACTORING.md

@@ -270,7 +270,7 @@ Track progress here. Mark tasks complete as they land and pass review.
 ### Phase 4 — Carve up the GUI
 - [x] T4.1 — Rename `gui` → `ui`; split app.go into run.go + mainwindow.go _(required Fyne v2.5.3→v2.6.3 upgrade for `fyne.Do`)_
 - [x] T4.2 — Extract `jobs_view.go`
-- [ ] T4.3 — Extract `job_dialog.go`
+- [x] T4.3 — Extract `job_dialog.go`
 - [ ] T4.4 — Extract `history_view.go`
 - [ ] T4.5 — Extract `settings_view.go`
 - [ ] T4.6 — Extract `tray.go`, `singleinstance.go`, `layout.go`