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-docker.sh

@@ -4,7 +4,7 @@ set -euo pipefail
 # Optional first argument mirrors build-linux.sh. The Docker build still writes
 # the final artifact into the local dist/ tree, not into the container. 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="${version:-0.0.0-dev}"
 tag="gitea.mixdep.ru/mix/gosentry-builder:${version}"
 output="${1:-dist/linux/gosentry-${version}-linux-amd64}"
@@ -26,7 +26,7 @@ docker run --rm \
     -v "$(pwd):/src" \
     -w /src \
     "$tag" \
-    bash -c 'CGO_ENABLED=1 GOOS=linux GOARCH=amd64 go build -buildvcs=false -trimpath -ldflags "-s -w -X gitea.mixdep.ru/mix/gosentry/src/core.Version=${VERSION}" -o "${OUTPUT}" ./cmd/gosentry'
+    bash -c 'CGO_ENABLED=1 GOOS=linux GOARCH=amd64 go build -buildvcs=false -trimpath -ldflags "-s -w -X gitea.mixdep.ru/mix/gosentry/src/app.Version=${VERSION}" -o "${OUTPUT}" ./cmd/gosentry'
 
 # Icons are embedded in the Go binary, so there is no assets directory to copy
 # after extracting the Linux executable.