Refactoring complete: v0.4.0 architectural milestone (#1)

## Summary

Completed Phase 5 refactoring and reached the target architecture.

**Architectural milestone achieved:**
- Service layer owns all state and is the sole writer
- UI is a thin Fyne view, all widget updates marshaled via `fyne.Do`
- Core engines are stateless and injectable
- Domain types are pure (no `yaml:"-"` fields)
- Full module builds and `go vet ./...` clean

## Changes

- Bump version: 0.3.6 → 0.4.0
- Update CHANGELOG with Phase 5 summary
- Add ROADMAP "Refactoring Follow-Ups" section

## Known follow-up work

1. **Linux test build broken** — `runner_test.go` needs `//go:build windows` tag
2. **File-size limits exceeded** — `operations.go` (486 lines), `jobs_view.go` (415 lines)

See ROADMAP.md for details.

---------

Co-authored-by: mixeme <mix.public@ya.ru>
Reviewed-on: #1

src/domain/job.go

@@ -0,0 +1,18 @@
+package domain
+
+// Job is the user-visible scheduled command. It contains only durable
+// configuration: every field is persisted to jobs.yaml. Transient execution
+// state (last run, next run, command output, in-memory activity) lives in a
+// separate JobRuntime so the jobs file stays a clean, hand-editable record of
+// configuration and never mixes in process-lifetime bookkeeping.
+type Job struct {
+	ID               int    `yaml:"id"`
+	Name             string `yaml:"name"`
+	Folder           string `yaml:"folder,omitempty"`
+	Schedule         string `yaml:"schedule"`
+	Command          string `yaml:"command"`
+	Arguments        string `yaml:"arguments,omitempty"`
+	SuccessExitCodes string `yaml:"success_exit_codes,omitempty"`
+	StartOnly        bool   `yaml:"start_only,omitempty"`
+	Enabled          bool   `yaml:"enabled"`
+}