fix: 3964 - progress texts instead of circular indicator for crop page + isolate

[monsieurtanuki]

What

• Now we display progress texts in the crop page instead of a circular progress indicator.
• This way we avoid the stuttering of the indicator during computing expensive operations like image+file
• In addition to that, we now use more compute instead of main thread computations.

Screenshot

with image	with progress text

Fixes bug(s)

• Fixes: Regression ? Image seems to be uploaded in main thread #3964

Impacted files

• app_en.arb: added 4 labels for the action progress on crop page
• background_task_image.dart: minor refactoring
• crop_page.dart: displays a text instead of a stuttering circular progress indicator; minor refactoring
• image_compute_container.dart: now using compute as much as possible - still not possible for ui

[codecov-commenter]

Codecov Report

Merging #3965 (2c1d3b8) into develop (cd04135) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           develop    #3965      +/-   ##
===========================================
- Coverage    11.00%   10.97%   -0.03%     
===========================================
  Files          265      265              
  Lines        13044    13070      +26     
===========================================
  Hits          1435     1435              
- Misses       11609    11635      +26     

Impacted Files	Coverage Δ
...ooth_app/lib/background/background_task_image.dart	0.00% <0.00%> (ø)
...mooth_app/lib/helpers/image_compute_container.dart	0.00% <0.00%> (ø)
packages/smooth_app/lib/pages/crop_page.dart	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

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[teolemon]

Is the cropping image… static ?

[monsieurtanuki]

Is the cropping image… static ?

@teolemon Not sure I get what you mean.
What I can say is that the full operation lasts about 2 seconds, and that when you click on the "send!" button the image is replaced by something like 4 messages, according on the progress.