feat(phase3): PWA, auto-discovery, bookmarklet, multi-tab sync

- PWA: manifest, service worker (cache-first static, network-first API),
  offline page, install prompt banner
- Auto-discovery: Docker socket + Traefik API scanning, approval UI
- Quick-add bookmarklet: popup-based add page, favicon auto-detect
- Multi-tab sync: BroadcastChannel for theme + data changes
- i18n translations for all new strings (EN/RU)

.env.example

@@ -24,5 +24,9 @@ GUEST_MODE="true"
 HEALTHCHECK_CRON="*/5 * * * *"
 HEALTHCHECK_TIMEOUT_MS="5000"
 
+# Service Discovery (optional — configure here or in Admin > Settings)
+DOCKER_SOCKET_PATH="/var/run/docker.sock"
+TRAEFIK_API_URL=""
+
 # Node environment
 NODE_ENV="production"