Follow-up to #8879, addressing its review.
## Changes
- **Community icons load the original again.** `AvatarImageContent`
gained a `preferThumbnail` flag, off by default. People and agent
avatars turn it on (as #8879 did for every caller); `CommunityAvatar`
leaves it off. Community logos are often transparent PNGs, and the
relay's JPEG thumbnail renders transparency black. This matches desktop,
which shows community icons from the original.
- **A missing thumbnail is recorded where its 404 arrives.** Before,
only the mounted `Image`'s error builder recorded it. If the avatar
scrolled away before a slow 404 came back, the thumbnail URL kept a 30s
failure cooldown instead, and showing that avatar again at any size
during the cooldown rendered initials and never requested the original.
The fetch now records a 404 on a relay `/media/<sha256>.thumb.jpg` URL
in the missing-thumbnail set and puts no cooldown on it.
## Tests
Two journey tests in `avatar_image_test.dart`, both failing on main and
passing here:
- An avatar unmounts before its slow thumbnail 404 arrives, then mounts
again at another size: it requests the original once, renders it (valid
image bytes, no initials), and leaves no cooldown.
- A 36pt `CommunityAvatar` with a relay-hosted icon requests and shows
only the original.
`flutter analyze` is clean. In the full local `flutter test` run, only
12 push and notification tests fail, and they fail the same way on main
in this environment.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
---------
Signed-off-by: Tom Brow <tomb@squareup.com>
Co-authored-by: Tom Brow <tomb@squareup.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
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