feat: Playnite library sync plugin + exporter

A two-part Punktfunk plugin that syncs the Playnite library into the host
game library as the `playnite` provider:

- exporter/ — a minimal C# Playnite GenericPlugin ("Punktfunk Sync") that
  writes punktfunk-library.json on every library change. Reading Playnite's
  live-locked LiteDB from outside isn't robust, so this adapter runs inside
  Playnite. Builds net462 via reference assemblies; packaged as a .pext.
- src/ — the TS plugin (on @punktfunk/host, rom-manager shape): watches the
  export, embeds covers as data URLs, reconciles via
  PUT /library/provider/playnite, with a console-hosted web UI + standalone
  fallback + CLI. Launch = playnite://playnite/start/<id>, run by the host
  in the interactive session so Playnite performs the real launch.

Verified locally: backend tsc + 23 tests + biome clean, SPA build, C#
exporter build + .pext pack.

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

src/art.ts

@@ -0,0 +1,60 @@
+// 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).
+
+import * as fs from "node:fs";
+import * as path from "node:path";
+import type { ArtOptions } from "./config.js";
+import type { ExportedGame } from "./export-schema.js";
+import type { Artwork } from "./wire.js";
+
+const MIME: Record<string, string> = {
+	".jpg": "image/jpeg",
+	".jpeg": "image/jpeg",
+	".png": "image/png",
+	".webp": "image/webp",
+	".gif": "image/gif",
+	".bmp": "image/bmp",
+	".ico": "image/x-icon",
+	".tga": "image/x-tga",
+};
+
+const mimeFor = (file: string): string | null =>
+	MIME[path.extname(file).toLowerCase()] ?? null;
+
+/** Read a local image file into a `data:` URL, or null if missing, oversized, or an unknown type. */
+export const fileToDataUrl = (
+	file: string | null,
+	maxBytes: number,
+): string | null => {
+	if (!file) return null;
+	const mime = mimeFor(file);
+	if (!mime) return null;
+	try {
+		const st = fs.statSync(file);
+		if (!st.isFile() || st.size === 0 || st.size > maxBytes) return null;
+		const b64 = fs.readFileSync(file).toString("base64");
+		return `data:${mime};base64,${b64}`;
+	} catch {
+		return null;
+	}
+};
+
+/** Build the artwork block for a game per the art config. Returns undefined when nothing was embedded. */
+export const buildArtwork = (
+	game: ExportedGame,
+	art: ArtOptions,
+): Artwork | undefined => {
+	if (art.mode === "off") return undefined;
+	const portrait = fileToDataUrl(game.cover, art.maxBytes);
+	const hero = art.includeBackground
+		? fileToDataUrl(game.background, art.maxBytes)
+		: null;
+	if (!portrait && !hero) return undefined;
+	const out: Artwork = {};
+	if (portrait) out.portrait = portrait;
+	if (hero) out.hero = hero;
+	return out;
+};