feat(nitro): convert imported SWF bundles to WebP Lossless

Newly converted .nitro bundles now store their spritesheet as WebP VP8L
instead of PNG, so imports land much smaller without changing a single
pixel. The texture member and spritesheet.meta.image are both labelled
from the actual bytes, never from a caller's assumption.

- encode through sharp with lossless and exact, so colour hidden under
  alpha 0 survives; this mirrors ImageSharp's TransparentColorMode.Preserve
- detect PNG/WebP by magic bytes and reject anything the client cannot
  render, on create, download and upload paths
- keep the source format when deriving size-32 sheets, scaling composites
  and editing metadata, so existing bundles are never silently rewritten
- report fidelity in the studio: the compression panel re-encodes with the
  same options the importer uses, so it cannot drift and invent false
  warnings, and shows PNG/WebP size estimates

convertSwfToNitro and buildSpritesheet are now async, so the worker, the
main-thread fallback and every import call site await them. PNG stays
supported for existing bundles and icon sidecars are untouched.

src/lib/services/import/core/download.ts

@@ -1,6 +1,9 @@
 import { promises as fs } from "node:fs";
 import { getByparrCookies } from "@/lib/services/byparr";
-import { parseNitroBundle } from "../../swf/nitro-builder";
+import {
+	detectNitroTextureFormat,
+	parseNitroBundle,
+} from "../../swf/nitro-builder";
 import { browserHeaders } from "./browser-headers";
 import { curlDownload } from "./curl-fetch";
 
@@ -27,7 +30,9 @@ export function validateNitroBytes(buffer: Buffer): boolean {
 			!!bundle.json &&
 			typeof bundle.json === "object" &&
 			!Array.isArray(bundle.json) &&
-			validatePngBytes(bundle.png)
+			// parseNitroBundle already rejects non-PNG/WebP textures, so this
+			// is a cheap re-assertion rather than the real gate.
+			detectNitroTextureFormat(bundle.texture) !== null
 		);
 	} catch {
 		return false;