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>

README.md

@@ -200,19 +200,18 @@ arty env/sync
 ### Toolkit (KIT System)
 
 ```bash
-# Access unified toolkit
-# Base URL: https://kit.pivoine.art
+# Access unified toolkit (subdomain routing)
 
-# File Converter (Vert) - /vert
-# URL: https://kit.pivoine.art/vert
+# File Converter (Vert)
+# URL: https://vert.kit.pivoine.art
 # Features:
 # - WebAssembly-based file conversion (250+ formats)
 # - Images, audio, documents, video
 # - Client-side processing (no uploads)
 # - No file size limits
 
-# Image Editor (Paint) - /paint
-# URL: https://kit.pivoine.art/paint
+# Image Editor (Paint)
+# URL: https://paint.kit.pivoine.art
 # Features:
 # - Browser-based image editing
 # - Layer support
@@ -275,7 +274,7 @@ THE FALCON (falcon_network)
 │  ├─ Linkwarden Marks   [links.pivoine.art]
 │  ├─ Vaultwarden Vault  [vault.pivoine.art]
 │  ├─ Joplin Sync Server [joplin.pivoine.art]
-│  ├─ Kit Toolkit        [kit.pivoine.art/vert, kit.pivoine.art/paint]
+│  ├─ Kit Toolkit        [vert.kit.pivoine.art, paint.kit.pivoine.art]
 │  ├─ Jellyfin Media     [jelly.pivoine.art]
 │  ├─ PairDrop Sharing   [drop.pivoine.art]
 │  ├─ Backrest Backups   [restic.pivoine.art]