feat(build): embed web UI in backend binary + simplify Docker variants (#620)

- Add internal/webui/ package with //go:build embedui tag for optional
  SPA embedding (handler.go serves static files with SPA fallback)
- Add internal/version/ shared semver comparison (DRY: extracted from
  gateway/update_check.go and updater/updater.go)
- Enhance UpdateChecker: release notes, ETag caching, filter lite-v* tags
- Add web UI build stage to Dockerfile with ENABLE_EMBEDUI build arg
- Simplify CI: 7 Docker variants → 4 (base, latest, full, otel)
- Add SHA256 checksums job to release workflow
- Add Makefile build-full target (embeds web UI in Go binary)
- Default make up now embeds web UI (no separate nginx needed)
- Add WITH_WEB_NGINX=1 flag for optional nginx reverse proxy
- Update README + 30 translated READMEs: make up, port 18790
- Update docker-compose comments and prepare-env.sh
- About dialog: show release notes with markdown rendering
- Health card: amber badge for available updates

BREAKING: Default Docker setup no longer requires selfservice overlay.
Web dashboard served at :18790 (same port as API).

scripts/install.sh

@@ -79,6 +79,8 @@ echo ""
 echo "GoClaw ${VERSION} installed to ${INSTALL_DIR}/goclaw"
 echo "Migrations installed to ${MIGRATIONS_DIR}"
 echo ""
+echo "The binary includes an embedded web dashboard — no separate nginx needed."
+echo ""
 echo "Next steps:"
 echo "  1. Set up PostgreSQL (pgvector):"
 echo "     docker run -d --name goclaw-pg -p 5432:5432 -e POSTGRES_PASSWORD=goclaw pgvector/pgvector:pg18"
@@ -92,3 +94,8 @@ echo "     goclaw onboard"
 echo ""
 echo "  4. Start the gateway:"
 echo "     source .env.local && goclaw"
+echo ""
+echo "  Web dashboard: http://localhost:18790"
+echo "  Health check:  curl http://localhost:18790/health"
+echo ""
+echo "  To update later: goclaw update --apply"