Fix setImage latency in mobile-fitting-room: resize garment images to max 1024px

[tomershlasky]

Summary

• The mobile-fitting-room example was calling setImage with the full-resolution blob from urlToImageBlob, while full-featured and outfit-builder both pipe through resizeImageBlob(blob, 1024) first. High-res garment images (e.g. 2000–4000px from fashion sites) cause significant switch latency.
• Added resizeImageBlob call in applyProduct and in the enterFitRoom initial image path.
• Updated the README best-practices section to document the 512–1024px recommendation for setImage and note that oversized images increase switch latency.

Root cause

Customer (SwapCommerce) reported ~6s delays switching outfits via set_image. Their example images were 530KB–1.5MB. The mobile-fitting-room example had the same bug: no image resize before setImage, unlike the other examples.

Requested by Jake Rothman in Slack (#swapcommerce--decart)

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