chrome: brandIcons() — derive Next.js favicon metadata from Brand

Every app that imports the kit was shipping a /favicon.ico 404
because none of them wired up Next.js's metadata.icons. This adds
a tiny helper so an app only has to:

  export const metadata: Metadata = {
    title: brand.product,
    icons: brandIcons(brand),
  };

brandIcons() returns icon/shortcut/apple entries pointing at
brand.faviconUrl (new optional field, defaults to brand.logoUrl).
MIME type inferred from the URL extension (svg/png/ico).

Brand gains the optional faviconUrl field. Existing apps that just
pass logoUrl keep working — they'll now render the logo as the
favicon by default. Apps that want a separate icon set
faviconUrl explicitly.

First consumer: gscSounds layout — verified /favicon.ico now
serves the proper icon and /icon.svg works too.

src/chrome/brandIcons.ts

@@ -0,0 +1,37 @@
+// Returns a Next.js Metadata `icons` object derived from a Brand.
+// Apps drop this into their root layout's `metadata` export to ship
+// the brand logo as the favicon — fixes the "favicon 404" every
+// consumer of the kit was shipping with.
+//
+//   import type { Metadata } from "next";
+//   import { brandIcons } from "@gsc/web-kit/chrome";
+//   import { brand } from "@/config/brand";
+//
+//   export const metadata: Metadata = {
+//     title: brand.product,
+//     icons: brandIcons(brand),
+//   };
+
+import type { Brand } from "./types";
+
+interface BrandMetaIcons {
+  icon: { url: string; type?: string }[];
+  shortcut?: { url: string; type?: string }[];
+  apple?: { url: string; type?: string }[];
+}
+
+export function brandIcons(brand: Brand): BrandMetaIcons {
+  const url = brand.faviconUrl ?? brand.logoUrl;
+  // Heuristic: trust the file extension to set the MIME type. Most
+  // brand logos in the GoSec assets bucket are SVG.
+  const type =
+    /\.svg(\?|$)/i.test(url) ? "image/svg+xml" :
+    /\.png(\?|$)/i.test(url) ? "image/png" :
+    /\.ico(\?|$)/i.test(url) ? "image/x-icon" :
+    undefined;
+  return {
+    icon: [{ url, ...(type ? { type } : {}) }],
+    shortcut: [{ url }],
+    apple: [{ url }],
+  };
+}