fix(imaging): normalise avatar figures before building URLs

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.

src/lib/imager.ts

@@ -7,6 +7,7 @@
 export type { AvatarOptions } from "@/types/admin";
 
 import type { SyntheticEvent } from "react";
+import { normalizeAvatarFigure } from "@/lib/avatar-figure";
 import type { AvatarOptions } from "@/types/admin";
 
 /**
@@ -46,7 +47,7 @@ export function getAvatarUrl(
 	} = options;
 
 	const params = new URLSearchParams({
-		figure,
+		figure: normalizeAvatarFigure(figure),
 		img_format,
 		effect: String(effect),
 	});