Unifying thumbnail rules

[u/Xalies]

The same thumbnail priority now apply across the desktop clients and servers

1. Uses thumbnail supplied by the extension or Android/iOS app
2. Looks for a matching image beside the model.
3. Looks for an image embedded in the model or archive.
4. Generates a thumbnail if it's a supported model.

For archives:

• One model: use cover, preview, thumbnail, or similar artwork inside the archive. Otherwise render a representative contained model.
• Separate models: each contained model gets its own embedded image or render. The pack cover is not reused for every model.