Show asset names instead of thumbnails if preview image is empty

## Summary

Add option to toggle thumbnails or asset names in the palette window, or fallback to name if thumbnail is empty.

## Problem / Motivation

Some prefabs have difficult to see or non existent previews, this would introduce a quality of life improvement for prefab structures where the preview isn't able to be displayed.

## Proposed Solution

v1.1.0 will introduce a thumbnail scale slider in the palette view. This could be modified to behave similarly to the native project window slider where the smallest size is a stacked list of asset names and the larger sizes are thumbnails.

## Alternatives Considered

Global toggle in the settings window that binarily changes the entire palette view to use either names or thumbnails,
or experiment with dynamically changing each preview box but I think this could end up looking messy and inconsistent.

Start with the palette window implementation and inspect how thumbnail previews are currently displayed. Compare the proposed thumbnail scale behavior with the native Project window, then define whether completion means a name fallback, a scale-controlled list mode, or a settings toggle, with the chosen behavior working for assets whose previews cannot be displayed.

Written by the indexing model from the issue text.