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

scripts/build-linux.sh

@@ -3,7 +3,7 @@ set -euo pipefail
 
 # Optional first argument lets a developer or CI job choose the output path. The
 # default includes the application version and target platform.
-version="$(sed -n 's/^var Version = "\(.*\)"/\1/p' src/core/version.go)"
+version="$(sed -n 's/^var Version = "\(.*\)"/\1/p' src/app/version.go)"
 version="${version:-0.0.0-dev}"
 output="${1:-dist/linux/gosentry-${version}-linux-amd64}"
 mkdir -p "$(dirname "$output")"
@@ -17,7 +17,7 @@ export GOARCH=amd64
 
 # -trimpath removes local machine paths from debug/build metadata. -s -w strips
 # symbol/debug tables to keep the desktop binary smaller.
-go build -trimpath -ldflags "-s -w -X gitea.mixdep.ru/mix/gosentry/src/core.Version=${version}" -o "$output" ./cmd/gosentry
+go build -trimpath -ldflags "-s -w -X gitea.mixdep.ru/mix/gosentry/src/app.Version=${version}" -o "$output" ./cmd/gosentry
 
 # The application icon is embedded by Go, so the Linux build does not need a
 # sidecar assets directory beside the executable.