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/config.go

@@ -0,0 +1,25 @@
+package domain
+
+// StartInTrayArgument is written to the Windows Startup shortcut so autostart
+// can keep the scheduler running without flashing the main window. Manual
+// launches omit this flag and open the normal window.
+const StartInTrayArgument = "--start-in-tray"
+
+// Config is stored in gosentry.yaml next to the program. It contains only
+// application-level choices: where to read jobs from, where to write logs, and
+// how the desktop shell should behave.
+type Config struct {
+	JobsDir           string `yaml:"jobs_dir"`
+	LogsDir           string `yaml:"logs_dir"`
+	MaxLogFiles       int    `yaml:"max_log_files"`
+	MaxLogAgeDays     int    `yaml:"max_log_age_days"`
+	StartOnLogin      bool   `yaml:"start_on_login"`
+	KeepRunningInTray bool   `yaml:"keep_running_in_tray"`
+	NotifyOnFailure   bool   `yaml:"notify_on_failure"`
+}
+
+// JobsFile is the on-disk shape of jobs.yaml. Wrapping the slice in a top-level
+// object leaves room for future metadata without breaking the basic file format.
+type JobsFile struct {
+	Jobs []Job `yaml:"jobs"`
+}