fix(ui): stop requesting image thumbnails larger than the original

[xsahil03x]

ThumbnailSizeCalculator sized a thumbnail request as the layout box fitted to the image's aspect ratio, multiplied by the device pixel ratio. originalSize was read only for that ratio and never used as a ceiling, so the request was min(originalPixels, boxLogical) × devicePixelRatio with no upper bound — any image smaller than the box times the pixel ratio was asked for at more pixels than it holds. On a 3x phone with a ~300pt bubble that is every image narrower than ~900px: screenshots, stickers, and anything already downscaled on upload.

Those extra pixels do not exist in the source, so they are interpolated. The requested size also reaches the CDN cache key and memCacheWidth/memCacheHeight, so each device that overshoots by a different amount downloads its own oversized rendition and decodes it into the image cache at that size.

The calculated size is now capped at the original.

feat/v9-image-size-ladder changes the same tail of calculate(); when it restacks, the cap must land after the snap (min(snap(w), original)) so the ladder cannot round a capped size back above the source.

Verification

flutter test on thumbnail_size_calculator_test.dart (22 passing), flutter analyze --fatal-infos and dart format clean on the changed files. The four previously-upscaling cases are now covered by tests that assert the original size is returned.

FLU-807

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (v9@e57f7c4). Learn more about missing BASE report.

Additional details and impacted files

@@          Coverage Diff          @@
##             v9    #2984   +/-   ##
=====================================
  Coverage      ?   67.42%           
=====================================
  Files         ?      431           
  Lines         ?    27449           
  Branches      ?        0           
=====================================
  Hits          ?    18508           
  Misses        ?     8941           
  Partials      ?        0           

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