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>

CLAUDE.md

@@ -251,10 +251,17 @@ Joplin Server note-taking and synchronization platform:
 5. Enter email and password created in step 1
 
 ### Kit (kit/compose.yaml)
-Unified toolkit combining file conversion and image editing using subdomain routing:
+Unified toolkit with landing page, file conversion, and image editing using subdomain routing:
+- **Landing**: `kit.pivoine.art` - Toolkit landing page
 - **Vert**: `vert.kit.pivoine.art` - Universal file format converter
 - **Paint**: `paint.kit.pivoine.art` - Web-based image editor
 
+#### Landing Page (kit.pivoine.art)
+Kit toolkit landing page:
+- Main entry point for the toolkit
+- Links to Vert and Paint services
+- Clean, simple interface
+
 #### Vert Service (vert.kit.pivoine.art)
 VERT universal file format converter:
 - WebAssembly-based file conversion (client-side processing)