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>

README.md

@@ -54,7 +54,7 @@ The **Falcon** is a state-of-the-art containerized starship, powered by Docker's
 | **LINKS** | *Interstellar bookmark archive* | [links.pivoine.art](https://links.pivoine.art) |
 | **VAULT** | *Encrypted password vault* | [vault.pivoine.art](https://vault.pivoine.art) |
 | **JOPLIN** | *Note-taking server & sync hub* | [joplin.pivoine.art](https://joplin.pivoine.art) |
-| **KIT** | *Toolkit: file converter & image editor* | [kit.pivoine.art](https://kit.pivoine.art) |
+| **KIT** | *Toolkit hub with converter & editor* | [kit.pivoine.art](https://kit.pivoine.art) |
 | **JELLY** | *Media streaming server* | [jelly.pivoine.art](https://jelly.pivoine.art) |
 | **DROP** | *Peer-to-peer file sharing* | [drop.pivoine.art](https://drop.pivoine.art) |
 | **RESTIC** | *Automated backup vault system* | [restic.pivoine.art](https://restic.pivoine.art) |
@@ -202,6 +202,10 @@ arty env/sync
 ```bash
 # Access unified toolkit (subdomain routing)
 
+# Landing Page
+# URL: https://kit.pivoine.art
+# Main entry point with links to all toolkit services
+
 # File Converter (Vert)
 # URL: https://vert.kit.pivoine.art
 # Features: