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/runtime.go

@@ -0,0 +1,49 @@
+package domain
+
+import "time"
+
+// JobRuntime is the transient execution state for a Job. It is never written to
+// jobs.yaml: it is rebuilt from scratch each time GoSentry starts and is held in
+// memory keyed by Job.ID for the lifetime of the process. Keeping it separate
+// from Job is what lets the durable configuration file stay free of run records,
+// status strings, and scheduling bookkeeping.
+type JobRuntime struct {
+	LastRun   string
+	NextRun   string
+	LastState string
+	Output    string
+	Logs      []RunRecord
+
+	// NextDue is the next scheduled execution time, kept as time.Time for
+	// scheduler comparisons. NextRun above is its formatted display string and is
+	// the only form shown in the GUI.
+	NextDue time.Time
+}
+
+// NewRuntime builds the initial runtime state for a freshly loaded or created
+// job. Enabled jobs start "Ready" and wait for the scheduler to compute their
+// first run; disabled jobs start "Paused".
+func NewRuntime(job Job) *JobRuntime {
+	runtime := &JobRuntime{
+		LastRun: "Never",
+		Output:  "No command output captured yet.",
+	}
+	if job.Enabled {
+		runtime.LastState = "Ready"
+		runtime.NextRun = "After start"
+	} else {
+		runtime.LastState = "Paused"
+		runtime.NextRun = "Paused"
+	}
+	return runtime
+}
+
+// NewRuntimes builds a runtime map for a slice of jobs, keyed by Job.ID. It is
+// the convenience entry point used when a whole jobs file has just been loaded.
+func NewRuntimes(jobs []Job) map[int]*JobRuntime {
+	runtimes := make(map[int]*JobRuntime, len(jobs))
+	for _, job := range jobs {
+		runtimes[job.ID] = NewRuntime(job)
+	}
+	return runtimes
+}