feat: 3656 - privacy compliance for cropped new images

[monsieurtanuki]

Impacted files:

• background_task_image.dart: now we crop here the image before sending it
• new_crop_page.dart: now we either let the server crop (old image) or let the background task crop (brand new image)
• rotated_crop_controller.dart: refactored adding a static method getCroppedBitmap

What

• There were privacy issues with image cropping, as cropped images were sent as full images with cropped parameters. That meant that areas beyond the crop were still visible, which can be problematic and is not a standard in apps.
• Above that, there are performance issues about cropping locally on "small" smartphones like @alexgarel's.
• @M123-dev suggested that we make a clear difference between brand new pictures (just taken by the users) and "repository" pictures (taken a while ago and allegedly privacy compliant).
• For brand new pictures, we send only the cropped area, and we compute it locally, inside the background task in order to avoid waiting too long after clicking on the "Send!" button.
• For "old" pictures, we let the server crop the already existing image - there's no change in the code in that case.
• An isolate could help with performances for local cropping, if stuttering UI is detected.

Fixes bug(s)

• Closes: Privacy review of server-side cropping #3656

[codecov-commenter]

Codecov Report

Merging #3673 (b311cca) into develop (bf1fe91) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           develop   #3673      +/-   ##
==========================================
- Coverage     9.45%   9.42%   -0.03%     
==========================================
  Files          269     269              
  Lines        13576   13611      +35     
==========================================
  Hits          1283    1283              
- Misses       12293   12328      +35     

Impacted Files	Coverage Δ
...ooth_app/lib/background/background_task_image.dart	0.00% <0.00%> (ø)
...th_app/lib/pages/image/uploaded_image_gallery.dart	0.00% <ø> (ø)
packages/smooth_app/lib/pages/image_crop_page.dart	0.00% <ø> (ø)
...th_app/lib/pages/product/product_image_viewer.dart	0.00% <ø> (ø)
...s/smooth_app/lib/tmp_crop_image/new_crop_page.dart	0.00% <0.00%> (ø)
...pp/lib/tmp_crop_image/rotated_crop_controller.dart	0.00% <0.00%> (ø)

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[M123-dev]

Looks great, thanks @monsieurtanuki

[M123-dev]

Could we still log this, in case something happens (same a few lines above)

[monsieurtanuki]

@M123-dev For this particular case (_getCroppedPath()) that wouldn't be such a good idea as sometimes - when the crop is actually the "full image" - we don't create this path/File. We would then have to handle a special flag for that.
Regarding the other cases, I wouldn't say it would be completely pointless, but almost: if it fails the rest of the app will work OK, including the next uploads.
And there's already an open issue regarding background task init+garbage collecting, that would get rid of those files anyway.
One day we may log this, but today that looks like a "just in case" that we don't really need.
Anyway sentry is overcrowded with camera issues that make it useless.

[monsieurtanuki]

Thank you @M123-dev for the review!