feat: show display names for Dockge agents

[NightWatcher314]

Summary

• allow Dockge agents to keep an optional display name and normalize empty values
• show the display name in the dashboard, stack group headers, and compose agent selector
• fall back to the endpoint when no display name is configured

Verification

• npm run check-ts
• npm run lint (passes with existing warnings)
• npm run build:frontend
• built and pushed a Docker image from this branch to a private registry for runtime packaging verification