chore: set tsup platform to node

Plugins run server-side during the Quartz build step, not in the
browser. Setting platform: 'node' tells esbuild to treat Node builtins
as available externals and avoids CJS-in-ESM require() failures for
any bundled dependencies that use Node APIs.

tsup.config.ts

@@ -14,6 +14,7 @@ export default defineConfig({
   target: "es2022",
   splitting: false,
   outDir: "dist",
+  platform: "node",
   esbuildOptions(options) {
     options.jsx = "automatic";
     options.jsxImportSource = "preact";