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/avatar-figure.ts

@@ -0,0 +1,38 @@
+/**
+ * Client-safe avatar figure rules, shared by the URL builder (`imager.ts`) and
+ * the server-side query parser (`imager-upstream.ts`).
+ *
+ * The imager is the authority on what it can draw: a figure with an empty part
+ * slot (e.g. `hr-100-.hd-180-1.ch-255-66`) is rejected with 400 "Invalid figure
+ * format". Legacy accounts can still carry such stored looks, so the URL
+ * builder normalises them to a valid default instead of emitting a request that
+ * renders as a broken image.
+ */
+
+/** Valid starter figure — also the default look new accounts are given. */
+export const DEFAULT_AVATAR_FIGURE =
+	"hr-100-1.hd-180-1.ch-255-66.lg-280-110.sh-305-62";
+
+/**
+ * The first segment must look like a real part (`hd-180`); the rest is
+ * deliberately loose because some clients append extra segments (e.g. a
+ * trailing `.0`) that the renderer still accepts. Over-strict validation here
+ * turned valid avatars into 400s.
+ */
+export const AVATAR_FIGURE_RE = /^[a-z]{2}-\d+(?:[.-][a-z0-9]+)*$/i;
+export const AVATAR_FIGURE_MAX_LEN = 512;
+export const AVATAR_FIGURE_MAX_PARTS = 24;
+
+/**
+ * Return `figure` unchanged when the imager can draw it, otherwise the default
+ * figure. Trims surrounding whitespace either way.
+ */
+export function normalizeAvatarFigure(figure: string): string {
+	const trimmed = figure.trim();
+	if (!trimmed || trimmed.length > AVATAR_FIGURE_MAX_LEN)
+		return DEFAULT_AVATAR_FIGURE;
+	if (!AVATAR_FIGURE_RE.test(trimmed)) return DEFAULT_AVATAR_FIGURE;
+	if (trimmed.split(".").length > AVATAR_FIGURE_MAX_PARTS)
+		return DEFAULT_AVATAR_FIGURE;
+	return trimmed;
+}