fix(manacore/web): add sync server URL build arg to Dockerfile

Production build was falling back to localhost:3050 for mana-sync because
PUBLIC_SYNC_SERVER_URL was not set as a build-time ARG. Vite bakes
import.meta.env vars at build time, so the runtime docker-compose
environment section alone is insufficient.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.macmini.yml

@@ -845,6 +845,8 @@ services:
     build:
       context: .
       dockerfile: apps/manacore/apps/web/Dockerfile
+      args:
+        PUBLIC_SYNC_SERVER_URL: wss://sync.mana.how
     image: manacore-web:local
     container_name: mana-app-web
     restart: always