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feat: compress generated thumbnails with png quantization (#3416)

the 1024x1024 `thumbnail.png` images generated for wearables and emotes are
heavy because they're encoded as lossless png. this re-encodes them with a
quantized (indexed) palette via `upng-js`, which preserves the alpha channel
(unlike jpeg/webp) while greatly reducing the file size of 3d-rendered images.

- add `compressPngBlob(blob, colors = 256)` in `modules/media/utils`. it has a
  size floor: if the quantized result isn't smaller (e.g. smooth gradients that
  quantize poorly), or decoding/encoding fails, the original blob is returned by
  reference, so it can never produce a larger or broken file.
- compress `thumbnail.png` in both save sagas (`handleSaveItemRequest`, which the
  reset path also uses, and the bulk `handleSaveMultipleItemsRequest`). uploads are
  content-hash-addressed, so the thumbnail's content hash is recomputed when the
  blob changes.
- compress the catalyst `image.png` inside `generateImage`, covering the save, bulk
  and deploy paths. its hash stays consistent because it's computed downstream.
- palette size is configurable via the `colors` argument / `THUMBNAIL_PALETTE_COLORS`.

note: this is a size floor, not a perceptual one. `upng-js` has no pngquant-style
quality threshold or dithering, so the gradient `image.png` is the asset most prone
to banding at 256 colors.

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