Rewrite composer image state management, remove MobX

[pfrazee]

Supersedes #3890 by @mary-ext.

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Original PR description:

Decided to do this separately from #3813, removes the only remaining MobX dependency from the app.

• Temporary image files are moved to {cacheDirectory}/bsky-composer, this folder is set to purge when the composer is closed. I feel this is a worthwhile approach compared to making sure that images are being purged properly every time it's being manipulated.
• As a side benefit, the new image models are immutable.
• ALF'd image alt text and edit dialog

Alt text dialog	Edit image dialog

• Working with image cropping libraries is just as wonderful as I remembered it to be, react-avatar-editor has been replaced with react-image-crop which is slightly more workable. Would've preferred one that also does fixed cropping but react-easy-crop is being finicky with initial states.
• The image cropper dialog for avatars and banners still exists, but it's been cleaned up slightly

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Additions

• Removed aspect ratio controls from the composer image editor. Not really needed now that there's a cropping box, and it wasn't working correctly.
• Fixes to the cropping modal used by UserAvatar and UserBanner

[render]

Your Render PR Server URL is https://social-app-pr-3925.onrender.com.

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[github-actions]

The Pull Request introduced fingerprint changes against the base commit: 55ac287

Fingerprint diff

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[github-actions]

Old size	New size	Diff
6.44 MB	6.44 MB	157 B (0.00%)

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[mary-ext]

The new modals might make it easy to accidentally lose work if one manages to overshoot their mouse cursor while moving/resizing the crop area around.

I'll have to update #3884 to get it to work for the new modals as well if we still want to close on backdrop click but not on the case I just mentioned