Avatars render as circles everywhere (AppHeader, PostCard, profile
page), so a square crop frame let users compose an image that looked
fine in the cropper and got visibly clipped (lost corners, off-center
faces) after upload.
Switched the crop frame to a perfect circle by setting borderRadius =
CROP_SIZE / 2 on both the frame and the overflow mask. Replaced the
four square corner markers with a single thin white ring overlay
around the circle. Output is still a 512×512 JPEG — the consumer-side
border-radius does the visual circle, so the underlying square is
storage-agnostic and re-usable if we ever surface a non-circular
avatar elsewhere.
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