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

cmd/gosentry/main.go

@@ -3,15 +3,15 @@ package main
 import (
 	"os"
 
-	"gitea.mixdep.ru/mix/gosentry/src/core"
-	"gitea.mixdep.ru/mix/gosentry/src/gui"
+	"gitea.mixdep.ru/mix/gosentry/src/domain"
+	"gitea.mixdep.ru/mix/gosentry/src/ui"
 )
 
 func main() {
 	// The executable entry point intentionally delegates all startup work to the
-	// GUI package. Keeping main small makes it easier to add platform-specific
+	// UI package. Keeping main small makes it easier to add platform-specific
 	// packaging later without mixing window setup, storage, and scheduler logic.
-	gui.Run(hasArgument(core.StartInTrayArgument))
+	ui.Run(hasArgument(domain.StartInTrayArgument))
 }
 
 func hasArgument(argument string) bool {