feat: add Kit landing page at kit.pivoine.art

Added kit-ui landing page as the main entry point for the toolkit:
- Image: ghcr.io/valknarness/kit-ui:latest
- URL: https://kit.pivoine.art
- Provides links to Vert (file converter) and Paint (image editor)

Changes:
- Added landing service to kit/compose.yaml
- Updated arty.yml with KIT_TRAEFIK_HOST and KIT_LANDING_IMAGE
- Updated CLAUDE.md with landing page documentation
- Updated README.md with landing page reference

Toolkit structure:
- Landing: kit.pivoine.art (main hub)
- Vert: vert.kit.pivoine.art (file converter)
- Paint: paint.kit.pivoine.art (image editor)

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

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

arty.yml

@@ -110,9 +110,11 @@ envs:
     JOPLIN_TRAEFIK_HOST: joplin.pivoine.art
     JOPLIN_APP_PORT: 22300
     JOPLIN_DB_NAME: joplin
-    # Kit (combines Vert and Paint)
+    # Kit (combines Vert and Paint with landing page)
     KIT_TRAEFIK_ENABLED: true
     KIT_COMPOSE_PROJECT_NAME: kit
+    KIT_TRAEFIK_HOST: kit.pivoine.art
+    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