refactor: remove paint service from kit stack

Removed miniPaint image editor service from the kit stack:
- Deleted paint service definition from kit/compose.yaml
- Removed Dockerfile for paint build
- Removed KIT_PAINT_TRAEFIK_HOST from arty.yml
- Stopped and removed kit_paint container

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

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

arty.yml

@@ -132,7 +132,6 @@ envs:
     KIT_LANDING_IMAGE: ghcr.io/valknarness/kit-ui:latest
     KIT_VERT_IMAGE: ghcr.io/vert-sh/vert:latest
     KIT_VERT_TRAEFIK_HOST: vert.kit.pivoine.art
-    KIT_PAINT_TRAEFIK_HOST: paint.kit.pivoine.art
     KIT_PASTEL_API_IMAGE: ghcr.io/valknarness/pastel-api:latest
     KIT_PASTEL_UI_IMAGE: ghcr.io/valknarness/pastel-ui:latest
     KIT_PASTEL_TRAEFIK_HOST: pastel.kit.pivoine.art