feat(thumbnails): split preview mime types + add HasPreview

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Split SupportedMimeTypes into UnconditionalPreviewMimeTypes (preview follows
from the mimetype) and EmbeddedPreviewMimeTypes (audio cover art, may be
absent); SupportedMimeTypes stays as their union for the generator. Add
HasPreview(md) as the single source of truth: unconditional types are always
true, embedded types depend on stored oc.preview dimensions.

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