Upgrade Next.js to v16 and refactor API client

Bump Next.js from v9 to v16.2.6 and add node-ical to
serverExternalPackages. Remove Geist font dependencies in favor
of system fonts. Fix state updates in geocode hooks to prevent
stale closures. Expose ApiClient methods as public and improve
error handling in fetchWithRetry.

next.config.ts

@@ -2,6 +2,7 @@ import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
   output: "standalone",
+  serverExternalPackages: ["node-ical"],
 };
 
 export default nextConfig;