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/schedule_test.go

@@ -0,0 +1,103 @@
+package domain
+
+import (
+	"testing"
+	"time"
+)
+
+func TestParseRejectsInvalidSchedules(t *testing.T) {
+	cases := []struct {
+		spec string
+		desc string
+	}{
+		{"", "empty string"},
+		{"   ", "whitespace only"},
+		{"@every", "bare @every without duration"},
+		{"@every ", "@every with trailing space but no duration"},
+		{"@every xyz", "invalid @every duration string"},
+		{"@every -1s", "negative @every duration"},
+		{"@every 0s", "zero @every duration"},
+		{"not-a-cron", "invalid cron expression"},
+		{"60 * * * *", "cron minute out of range"},
+		{"* * * *", "too few cron fields"},
+	}
+	for _, tc := range cases {
+		if _, err := Parse(tc.spec); err == nil {
+			t.Errorf("Parse(%q) [%s]: expected error, got nil", tc.spec, tc.desc)
+		}
+		if err := Validate(tc.spec); err == nil {
+			t.Errorf("Validate(%q) [%s]: expected error, got nil", tc.spec, tc.desc)
+		}
+	}
+}
+
+func TestParseEveryInterval(t *testing.T) {
+	from := time.Date(2026, 6, 14, 12, 0, 0, 0, time.UTC)
+	s, err := Parse("@every 10s")
+	if err != nil {
+		t.Fatalf("Parse(@every 10s): unexpected error: %v", err)
+	}
+	if got, want := s.Next(from), from.Add(10*time.Second); !got.Equal(want) {
+		t.Fatalf("Next: got %s, want %s", got, want)
+	}
+}
+
+func TestParseEveryTrimsSurroundingWhitespace(t *testing.T) {
+	from := time.Date(2026, 6, 14, 12, 0, 0, 0, time.UTC)
+	s, err := Parse("  @every 90m  ")
+	if err != nil {
+		t.Fatalf("Parse: unexpected error: %v", err)
+	}
+	if got, want := s.Next(from), from.Add(90*time.Minute); !got.Equal(want) {
+		t.Fatalf("Next: got %s, want %s", got, want)
+	}
+}
+
+func TestParseCronExpression(t *testing.T) {
+	from := time.Date(2026, 6, 14, 12, 3, 0, 0, time.UTC)
+	s, err := Parse("*/5 * * * *")
+	if err != nil {
+		t.Fatalf("Parse(*/5 * * * *): unexpected error: %v", err)
+	}
+	want := time.Date(2026, 6, 14, 12, 5, 0, 0, time.UTC)
+	if got := s.Next(from); !got.Equal(want) {
+		t.Fatalf("Next: got %s, want %s", got, want)
+	}
+}
+
+func TestParseCronDescriptor(t *testing.T) {
+	from := time.Date(2026, 6, 14, 12, 3, 0, 0, time.UTC)
+	s, err := Parse("@daily")
+	if err != nil {
+		t.Fatalf("Parse(@daily): unexpected error: %v", err)
+	}
+	want := time.Date(2026, 6, 15, 0, 0, 0, 0, time.UTC)
+	if got := s.Next(from); !got.Equal(want) {
+		t.Fatalf("Next: got %s, want %s", got, want)
+	}
+}
+
+func TestValidateAcceptsValidSchedules(t *testing.T) {
+	for _, spec := range []string{"@every 1s", "*/5 * * * *", "0 9 * * 1", "@hourly"} {
+		if err := Validate(spec); err != nil {
+			t.Errorf("Validate(%q): unexpected error: %v", spec, err)
+		}
+	}
+}
+
+func TestZeroScheduleNextIsZero(t *testing.T) {
+	var s Schedule
+	if got := s.Next(time.Now()); !got.IsZero() {
+		t.Fatalf("zero Schedule Next: got %s, want zero time", got)
+	}
+}
+
+func TestStringReturnsTrimmedSpec(t *testing.T) {
+	s, err := Parse("  */5 * * * *  ")
+	if err != nil {
+		t.Fatalf("Parse: unexpected error: %v", err)
+	}
+	if got, want := s.String(), "*/5 * * * *"; got != want {
+		t.Fatalf("String: got %q, want %q", got, want)
+	}
+}