refactor: switch Kit stack from path to subdomain routing

Changed from path-based routing (kit.pivoine.art/vert, /paint) to
subdomain routing (vert.kit.pivoine.art, paint.kit.pivoine.art) to
fix CSS and JS asset loading issues.

Changes:
- Updated kit/compose.yaml: Removed path prefix stripping, using Host() rules
- Updated arty.yml: Added KIT_VERT_TRAEFIK_HOST and KIT_PAINT_TRAEFIK_HOST
- Updated CLAUDE.md: Changed URLs from paths to subdomains
- Updated README.md: Updated all Kit references to use subdomains

New URLs:
- File converter: https://vert.kit.pivoine.art
- Image editor: https://paint.kit.pivoine.art

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

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

arty.yml

@@ -113,8 +113,9 @@ envs:
     # Kit (combines Vert and Paint)
     KIT_TRAEFIK_ENABLED: true
     KIT_COMPOSE_PROJECT_NAME: kit
-    KIT_TRAEFIK_HOST: kit.pivoine.art
     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
     # Jellyfin
     JELLY_TRAEFIK_ENABLED: true
     JELLY_COMPOSE_PROJECT_NAME: jelly