feat: add Excalidraw to Kit stack

Added Excalidraw collaborative drawing tool to the Kit toolkit at draw.kit.pivoine.art:
- Virtual whiteboard for sketching hand-drawn diagrams
- Infinite canvas with collaborative features
- Image: excalidraw/excalidraw:latest
- Exposed on port 80 via Traefik with SSL

Configuration:
- Added draw service to kit/compose.yaml
- Added KIT_DRAW_IMAGE and KIT_DRAW_TRAEFIK_HOST to arty.yml
- Health check disabled to prevent false negatives
- Follows same pattern as other Kit tools
- Watchtower enabled for automatic updates

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

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

arty.yml

@@ -125,6 +125,8 @@ envs:
     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
+    KIT_DRAW_IMAGE: excalidraw/excalidraw:latest
+    KIT_DRAW_TRAEFIK_HOST: draw.kit.pivoine.art
     # Jellyfin
     JELLY_TRAEFIK_ENABLED: true
     JELLY_COMPOSE_PROJECT_NAME: jelly