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>

src/art.ts

@@ -1,8 +1,8 @@
-// Cover art. Playnite stores art as local files on the host; the host's library only proxies Steam
-// art, and provider entries are fetched by clients directly — so for a Playnite title we inline the
-// cover as a `data:` URL the clients render without any network fetch. A `data:` URL bloats the
-// library payload, so anything over `maxBytes` is dropped rather than embedded (design default: cover
-// only; backgrounds are large and opt-in).
+// Cover art. Playnite stores art as local files on the host. Two delivery modes (see ArtMode):
+//  - `host` (default): emit the local file PATH; the updated host serves the bytes via its art proxy
+//    (`/library/art/…`), so the reconcile payload carries no image data and scales to any library size.
+//  - `dataurl`: inline the cover as a size-capped `data:` URL (self-contained, but only fit for small
+//    libraries — a big one blows past the host's reconcile body limit). `maxBytes` caps a single image.
 
 import * as fs from "node:fs";
 import * as path from "node:path";
@@ -48,6 +48,16 @@ export const buildArtwork = (
 	art: ArtOptions,
 ): Artwork | undefined => {
 	if (art.mode === "off") return undefined;
+	// `host` mode: emit the local file paths verbatim (the exporter only sets a path when the file
+	// exists). The host serves the bytes via its art proxy, so the reconcile payload carries no image
+	// data and scales to any library size. The default.
+	if (art.mode === "host") {
+		const out: Artwork = {};
+		if (game.cover) out.portrait = game.cover;
+		if (art.includeBackground && game.background) out.hero = game.background;
+		return out.portrait || out.hero ? out : undefined;
+	}
+	// `dataurl` mode: inline (size-capped) — self-contained but only fit for small libraries.
 	const portrait = fileToDataUrl(game.cover, art.maxBytes);
 	const hero = art.includeBackground
 		? fileToDataUrl(game.background, art.maxBytes)