Adopt new Asset 4 brand icon for favicon and small mark

Swap the favicon, Starlight logo/icon, and header/footer brand mark to the
new rounded-square Inky icon (blue→violet gradient). Regenerate the PNG
favicon fallback and bump cache-bust versions to force browsers to refresh.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SharonHart

astro.config.mjs

@@ -34,7 +34,7 @@ export default defineConfig({
         },
         { tag: "link", attrs: { rel: "preconnect", href: "https://fonts.googleapis.com" } },
         { tag: "link", attrs: { rel: "preconnect", href: "https://fonts.gstatic.com", crossorigin: true } },
-        { tag: "link", attrs: { rel: "icon", type: "image/svg+xml", href: "/inkey-icon.svg?v=2" } },
+        { tag: "link", attrs: { rel: "icon", type: "image/svg+xml", href: "/inkey-icon.svg?v=asset4" } },
         { tag: "link", attrs: { rel: "apple-touch-icon", href: "/apple-touch-icon.png?v=2" } },
         {
           tag: "link",