Split the server API module into client, guards, form fields and action helpers

web-player/src/lib/server/forms/fields.ts

@@ -0,0 +1,8 @@
+export function formString(form: FormData, key: string): string {
+	return String(form.get(key) ?? '');
+}
+
+export function formFile(form: FormData, key: string): File | null {
+	const value = form.get(key);
+	return value instanceof File && value.size > 0 ? value : null;
+}