feat(art): default to host-served cover art (scales to large libraries)

Adds art.mode "host" (now the default): emit each cover's local file PATH
and let the host serve the bytes via its art proxy, instead of inlining a
data URL. The reconcile payload carries no image data, so a large Playnite
library no longer exceeds the host's request-body limit. "dataurl" (inline,
small libraries) and "off" remain. Requires a host with the provider-art
proxy (punktfunk main: feat/library-local-art).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

config.example.json

@@ -6,7 +6,7 @@
 		"excludeSources": ["Xbox"]
 	},
 	"art": {
-		"mode": "dataurl",
+		"mode": "host",
 		"maxBytes": 1500000,
 		"includeBackground": false
 	},