fix: resize large images attachments on upload

[zaidhaan]

Mastodon's API has a 4096x4096px file size limit on image attachments, its web client however simply resizes larger images on the client-side to an acceptable size for the API request.

This PR brings this functionality to elk. The approach is based on theirs, though re-written by myself in a much more stripped down way.

To test, try uploading a large attachment like this one.

Fixes #1702

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[zaidhaan]

Hmm, works fine in dev locally but it looks like the Content Service Policy is blocking image blobs in the preview.

Also the error message doesn't look too great, yikes. Error handling could be improved

Shouldn't take too long to fix so I won't turn this into a draft just yet.

[zaidhaan]

Okay, all done.

CSP blob issue fixed. Resizing works.

As for the error message, per my last commit I just made it such that if resizing fails for whatever reason, then just attempt to upload the original file anyway. The user need not know about the details of a resizing failure, but it will still be logged in the console.

[patak-dev]

Awesome work @zaidhaan, let's merge it so we get some more testing on https://main.elk.zone