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).

docker-compose.postgres.yml

@@ -1,7 +1,8 @@
 # PostgreSQL overlay — pgvector for multi-tenant storage.
 #
-# Usage (with Web Dashboard):
-#   docker compose -f docker-compose.yml -f docker-compose.postgres.yml -f docker-compose.selfservice.yml up -d --build
+# Usage (web dashboard built-in by default):
+#   make up
+#   # or: docker compose -f docker-compose.yml -f docker-compose.postgres.yml up -d --build
 #
 # Required env vars (set in .env or shell):
 #   GOCLAW_OPENROUTER_API_KEY (or another provider key)