Theme build now rejects folders where a file is required

When a theme config entry (thumbnail, or a webpack/copy resource src) pointed to a folder instead of a file, the build succeeded but publishing via pnpm start/pnpm deploy later failed with a confusing HTTP 409 Conflict. The theme build now validates these paths early and fails immediately with a clear error naming the offending path.

(CMS-30250)

todo

todo

(CMS-21511)