fix(build): extract FAVICON_SERVICE_URL to prevent server-side createContext error

The share/[id] layout is a server component that imported FAVICON_SERVICE_URL
from icons.tsx, pulling in the entire React icon registry and triggering
createContext on the server. Moved the constant to its own favicon.ts module.

lib/utils/favicon.ts

@@ -0,0 +1,8 @@
+/**
+ * Google's public favicon service base URL.
+ * Append `?domain=<host>&sz=<px>` to get a favicon.
+ *
+ * Kept in a separate module so server components can import it
+ * without pulling in the React-dependent icon registry.
+ */
+export const FAVICON_SERVICE_URL = 'https://www.google.com/s2/favicons'