Replace the flutter_boring_avatars fallback with a friendly DiceBear
"thumbs" face, generated offline (dicebear_core) and deterministically
from the pubkey, rendered as SVG (flutter_svg). The thumbs style is
CC0 1.0 (public domain) — no attribution, AGPL-clean — unlike Multiavatar
whose custom licence restricts use. Style parsed once and per-pubkey SVG
memoized; the CC0 <metadata> block is stripped to keep flutter_svg quiet.
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