feat: add Units converter tool to Kit stack

Added Units converter to the Kit toolkit at units.kit.pivoine.art:
- Unit conversion tool for length, weight, temperature, etc.
- Image: ghcr.io/valknarness/units-ui:latest
- Exposed on port 3000 via Traefik with SSL

Configuration:
- Added units service to kit/compose.yaml
- Added KIT_UNITS_IMAGE and KIT_UNITS_TRAEFIK_HOST to arty.yml
- Follows same pattern as other Kit tools (Vert, Paint, Pastel, Stirling)
- Watchtower enabled for automatic updates

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

arty.yml

@@ -123,6 +123,8 @@ envs:
     KIT_PASTEL_TRAEFIK_HOST: pastel.kit.pivoine.art
     KIT_STIRLING_IMAGE: frooodle/s-pdf:latest
     KIT_STIRLING_TRAEFIK_HOST: stirling.kit.pivoine.art
+    KIT_UNITS_IMAGE: ghcr.io/valknarness/units-ui:latest
+    KIT_UNITS_TRAEFIK_HOST: units.kit.pivoine.art
     # Jellyfin
     JELLY_TRAEFIK_ENABLED: true
     JELLY_COMPOSE_PROJECT_NAME: jelly