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>

test/art.test.ts

@@ -58,25 +58,51 @@ describe("buildArtwork", () => {
 			buildArtwork(game({ cover }), { ...DEFAULT_ART, mode: "off" }),
 		).toBeUndefined();
 	});
-	test("dataurl mode embeds the cover as portrait", () => {
+	test("host mode (default) emits the raw cover path — no bytes", () => {
 		const cover = write("c2.png", Buffer.from([1, 2]));
 		const art = buildArtwork(game({ cover }), DEFAULT_ART);
+		expect(art?.portrait).toBe(cover);
+		expect(art?.hero).toBeUndefined();
+	});
+	test("host mode includeBackground adds the hero path", () => {
+		const cover = write("c2b.png", Buffer.from([1]));
+		const background = write("b2b.jpg", Buffer.from([2]));
+		const art = buildArtwork(game({ cover, background }), {
+			...DEFAULT_ART,
+			includeBackground: true,
+		});
+		expect(art?.portrait).toBe(cover);
+		expect(art?.hero).toBe(background);
+	});
+	test("host mode with no cover yields undefined", () => {
+		expect(buildArtwork(game({ cover: null }), DEFAULT_ART)).toBeUndefined();
+	});
+	test("dataurl mode embeds the cover as a portrait data URL", () => {
+		const cover = write("c3.png", Buffer.from([1, 2]));
+		const art = buildArtwork(game({ cover }), {
+			...DEFAULT_ART,
+			mode: "dataurl",
+		});
 		expect(art?.portrait?.startsWith("data:image/png;base64,")).toBe(true);
 		expect(art?.hero).toBeUndefined();
 	});
-	test("includeBackground adds hero", () => {
-		const cover = write("c3.png", Buffer.from([1]));
-		const background = write("b3.jpg", Buffer.from([2]));
+	test("dataurl includeBackground adds a hero data URL", () => {
+		const cover = write("c4.png", Buffer.from([1]));
+		const background = write("b4.jpg", Buffer.from([2]));
 		const art = buildArtwork(game({ cover, background }), {
 			...DEFAULT_ART,
+			mode: "dataurl",
 			includeBackground: true,
 		});
 		expect(art?.portrait).toBeTruthy();
 		expect(art?.hero?.startsWith("data:image/jpeg;base64,")).toBe(true);
 	});
-	test("no readable art yields undefined", () => {
+	test("dataurl mode with no readable art yields undefined", () => {
 		expect(
-			buildArtwork(game({ cover: "/does/not/exist.png" }), DEFAULT_ART),
+			buildArtwork(game({ cover: "/does/not/exist.png" }), {
+				...DEFAULT_ART,
+				mode: "dataurl",
+			}),
 		).toBeUndefined();
 	});
 });