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/storage/paths.go

@@ -0,0 +1,54 @@
+package storage
+
+import (
+	"os"
+	"path/filepath"
+)
+
+const (
+	// The config file stays beside the executable so the portable build behaves
+	// predictably: moving the program folder moves its settings with it.
+	ConfigFileName = "gosentry.yaml"
+	// Older builds were named PySentry. Keep the old config name readable during
+	// the rename window so portable installations can start once and rewrite the
+	// settings to gosentry.yaml without manual file copying.
+	LegacyConfigFileName = "pysentry.yaml"
+	// Jobs are kept in a separate YAML file because the user can choose a
+	// different jobs directory, while application settings remain local to the
+	// installed/copied program.
+	JobsFileName = "jobs.yaml"
+)
+
+// Paths contains both the physical program location and the resolved runtime
+// storage locations. Keeping resolved paths in one struct prevents the GUI and
+// scheduler from interpreting relative directories differently.
+type Paths struct {
+	ExecutablePath string
+	AppDir         string
+	ConfigPath     string
+	JobsDir        string
+	JobsPath       string
+	LogsDir        string
+	DesktopIcon    string
+}
+
+func ResolvePaths() (Paths, error) {
+	// os.Executable is used instead of the current working directory because GUI
+	// apps are often launched from Explorer, a tray shortcut, or a desktop file.
+	// In those cases the working directory can be surprising, but the executable
+	// path is stable and matches the "portable app folder" storage model.
+	executable, err := os.Executable()
+	if err != nil {
+		return Paths{}, err
+	}
+
+	appDir := filepath.Dir(executable)
+	configPath := filepath.Join(appDir, ConfigFileName)
+	return Paths{
+		ExecutablePath: executable,
+		AppDir:         appDir,
+		ConfigPath:     configPath,
+		JobsDir:        appDir,
+		JobsPath:       filepath.Join(appDir, JobsFileName),
+	}, nil
+}