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/ui/tray.go

@@ -0,0 +1,40 @@
+package ui
+
+import (
+	"fyne.io/fyne/v2"
+	fynedesktop "fyne.io/fyne/v2/driver/desktop"
+)
+
+func configureSystemTray(a fyne.App, w fyne.Window) {
+	desk, ok := a.(fynedesktop.App)
+	if !ok {
+		// Not every Fyne driver exposes desktop tray features. Returning silently
+		// keeps the same binary usable on platforms or sessions without a tray.
+		return
+	}
+
+	// IsQuit marks this as the tray's quit item. Without it Fyne's
+	// addMissingQuitForMenu appends a second, localized Quit (e.g. "Выход" on a
+	// Russian system) because it only recognizes an existing quit by matching the
+	// localized label — which our literal "Quit" does not. Setting IsQuit makes
+	// Fyne reuse this item instead of adding a duplicate, regardless of locale.
+	quit := fyne.NewMenuItem("Quit", func() {
+		a.Quit()
+	})
+	quit.IsQuit = true
+	menu := fyne.NewMenu("GoSentry",
+		fyne.NewMenuItem("Show", func() {
+			w.Show()
+			w.RequestFocus()
+		}),
+		fyne.NewMenuItemSeparator(),
+		quit,
+	)
+	desk.SetSystemTrayMenu(menu)
+	w.SetCloseIntercept(func() {
+		// Closing hides the window instead of quitting because scheduler tools are
+		// expected to keep working in the background. The explicit Quit tray item
+		// remains the way to stop the process.
+		w.Hide()
+	})
+}