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.test.ts

@@ -3,6 +3,7 @@ import { mkdtempSync, readdirSync, rmSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { DEFAULT_AVATAR_FIGURE } from "./avatar-figure";
 import { getAvatarUrl } from "./imager";
 import {
 	FALLBACK_TIMEOUT_MS,
@@ -52,6 +53,11 @@ describe("getAvatarUrl", () => {
 		expect(url).not.toContain("&x=1");
 	});
 
+	it("falls back to the default when the stored look is malformed", () => {
+		const url = getAvatarUrl("hr-100-.hd-180-1.ch-255-66.lg-280-110.sh-305-62");
+		expect(url).toContain(`figure=${DEFAULT_AVATAR_FIGURE}`);
+	});
+
 	it("allows custom img_format", () => {
 		const url = getAvatarUrl("hd-180-1", { img_format: "png" });
 		expect(url).toContain("img_format=png");