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.selfservice.yml

@@ -1,9 +1,14 @@
-# Self-service overlay — adds web dashboard UI (nginx + React SPA).
+# Self-service overlay — adds separate nginx web dashboard UI.
 #
-# Recommended usage:
-#   docker compose -f docker-compose.yml -f docker-compose.postgres.yml -f docker-compose.selfservice.yml up -d --build
+# OPTIONAL: The default `latest` image already includes an embedded web dashboard
+# served at port 18790. Use this overlay only if you need a dedicated nginx
+# reverse proxy for SSL termination, custom caching, or load balancing.
 #
-# Dashboard: http://localhost:3000
+# Usage:
+#   docker compose -f docker-compose.yml -f docker-compose.postgres.yml -f docker-compose.selfservice.yml up -d
+#   # or: make up WITH_WEB_NGINX=1
+#
+# Dashboard via nginx: http://localhost:3000
 
 services:
   goclaw-ui: