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/singleinstance.go

@@ -0,0 +1,64 @@
+package ui
+
+import (
+	"io"
+	"net"
+	"strings"
+	"time"
+
+	"fyne.io/fyne/v2"
+)
+
+const singleInstanceAddress = "127.0.0.1:37653"
+const singleInstanceShowCommand = "show"
+
+func acquireSingleInstance(showExisting bool) (net.Listener, bool) {
+	listener, err := net.Listen("tcp", singleInstanceAddress)
+	if err == nil {
+		return listener, true
+	}
+
+	connection, dialErr := net.DialTimeout("tcp", singleInstanceAddress, time.Second)
+	if dialErr == nil {
+		// The first instance listens only on localhost and understands one tiny
+		// command: "show". That keeps the implementation dependency-free and easy
+		// to inspect, which matters more here than introducing a named-pipe or
+		// platform-specific IPC abstraction just to focus an existing window.
+		if showExisting {
+			_, _ = io.WriteString(connection, singleInstanceShowCommand)
+		}
+		_ = connection.Close()
+		return nil, false
+	}
+
+	// If the port is unavailable but does not answer as GoSentry, continue
+	// startup instead of making the application impossible to open because of an
+	// unrelated local listener. In the normal duplicate-start case the dial above
+	// succeeds and this process exits after waking the first instance.
+	return nil, true
+}
+
+func serveSingleInstance(listener net.Listener, w fyne.Window) {
+	if listener == nil {
+		return
+	}
+	go func() {
+		for {
+			connection, err := listener.Accept()
+			if err != nil {
+				return
+			}
+			command, _ := io.ReadAll(io.LimitReader(connection, 32))
+			_ = connection.Close()
+			if strings.TrimSpace(string(command)) != singleInstanceShowCommand {
+				continue
+			}
+			// Accept runs on its own goroutine, so focusing the window must be
+			// marshaled onto the main thread like every other widget update.
+			fyne.Do(func() {
+				w.Show()
+				w.RequestFocus()
+			})
+		}
+	}()
+}