Flat channel listing doesn't always surface the avatar, so it could be stored
null → the orb fell back to plain initials (looked like a missing poster). Two
fixes:
- Orb falls back to the channel's newest video thumbnail when the avatar is
absent, so it's never blank.
- Opening the channel page (which resolves the real avatar) now backfills it
onto every wished row via set_wishlist_channel_poster — so the actual channel
avatar appears on the wishlist orb thereafter. 6 tests green.
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