feat: branding furni image position + scale (MPU background editor)

Renderer support for the in-client image position editor:
- FurnitureBrandedImageVisualization applies offsetX/Y to the branded image
  layer only (offsetZ stays as z-index/depth), so the image can be moved
  without shifting the furni frame
- new `scale` branding key + FURNITURE_BRANDING_SCALE: zooms the image via a
  real per-sprite scale (RoomObjectSprite.scale, default 1, applied in
  RoomSpriteCanvas) — NOT by writing the read-only width/height
- AssetManager loads external raster images (png/jpg/…) via a CORS <img> +
  Texture.from instead of Assets.load (which didn't load cross-origin images);
  branding image download failures are now surfaced instead of swallowed

packages/api/src/room/object/visualization/IRoomObjectSprite.ts

@@ -11,6 +11,7 @@ export interface IRoomObjectSprite
     texture: Texture;
     width: number;
     height: number;
+    scale: number;
     offsetX: number;
     offsetY: number;
     flipH: boolean;