ImagePicker.launchImageLibraryAsync now opens with `allowsEditing: true`
and `aspect: [1, 1]`, which triggers Apple's built-in square crop UI
(pan + zoom on the user's selection). The output URI is the actually
cropped image — fixing the long-standing bug where AvatarCropSheet
displayed a visual transform but `manipulateAsync` only resized the
original, so any pan/zoom the user did was discarded on confirm.
Removes the entire AvatarCropSheet component (~285 lines) and its sole
consumer wiring in profile/edit.tsx. The avatar continues to render as
a circle everywhere via borderRadius — the underlying square output is
just storage-agnostic.
Native-look-first per memory rule, zero new dependencies, no new
native module to link.
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