perf: byte budget for image cache, fix video thumbnail rebuild, clear byte cache on room exit

[9clg6]

Summary

Three targeted memory improvements following the perf audit. No functional changes — all existing behavior is preserved.

1. Byte budget for MxcImageCacheManager (50 MB)

The cache was limited to 100 entries with no size awareness. 100 images at 5 MB each = 500 MB of Dart heap. Now tracks _totalBytes and evicts LRU entries when either the count (100) or byte budget (50 MB) is exceeded.

// Before: count-only eviction
while (_imageCache.length >= _size) { _imageCache.remove(...); }

// After: count + byte budget eviction
while (_imageCache.length >= _maxCount ||
    (_totalBytes + imageData.length > _maxBytes && _imageCache.isNotEmpty)) {
  _totalBytes -= _imageCache[oldest]!.length;
  _imageCache.remove(oldest);
}

Also adds clear() method used by point 3 below.

2. Fix video thumbnail re-generation on every rebuild

_ImageWidget was a StatelessWidget whose build() called:

FutureBuilder(future: event?.room.generateVideoThumbnail(matrixVideoFile), ...)

Every setState() from a parent created a new Future, re-running generateVideoThumbnail() — which writes a temp file (mobile) or creates a blob URL (web), calls VideoThumbnail.thumbnailData(), and decodes the bitmap.

Fix: convert to StatefulWidget, compute the Future once in initState() and refresh in didUpdateWidget() only when data or event changes.

3. Clear raw byte cache on room exit

chat.dart dispose() already called PaintingBinding.instance.imageCache.clear() to evict decoded bitmaps. The compressed byte cache (MxcImageCacheManager) was never cleared — bytes accumulated across room navigations indefinitely. Now cleared alongside the decoded bitmap cache.

Test plan

• fvm flutter analyze clean
• Navigate between rooms — video thumbnails display correctly, no repeated network/decode calls
• Memory in Chrome DevTools stable after leaving rooms (heap does not grow linearly with room count)
• Images in chat display correctly after cache eviction

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[codescene-delta-analysis]

Gates Passed
3 Quality Gates Passed

See analysis details in CodeScene

Quality Gate Profile: The Bare Minimum
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[github-actions]

This PR has been deployed to https://linagora.github.io/twake-on-matrix/2987