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/app/api/admin/import/furni/batch-regen/route.ts

@@ -199,7 +199,10 @@ export const POST = withAdmin(
 						});
 						try {
 							const swfBuffer = await fs.readFile(swfPath);
-							const conversion = convertSwfToNitro(swfBuffer, baseClassname);
+							const conversion = await convertSwfToNitro(
+								swfBuffer,
+								baseClassname,
+							);
 							await fs.writeFile(nitroPath, conversion.bundle);
 
 							// Build FurnitureData entry if missing