Legacy accounts can carry a malformed stored look — the old register default
`hr-100-.hd-180-1...` has an empty colour slot — which the imager rejects with
400 "Invalid figure format", so the avatar renders broken everywhere the look
appears. Normalise the figure in the shared URL builder (`getAvatarUrl`, used by
every avatar on the site): an unrenderable look becomes `DEFAULT_AVATAR_FIGURE`
while the proxy route keeps rejecting malformed input with 400.
The rule set and default now live in one client-safe module
(`src/lib/avatar-figure.ts`), shared by the URL builder and the server parser.
