refactor: centralize validation, error handling, and formatting logic

Extract shared auth logic and validation rules to shared/auth.ts
Introduce utility functions for HTTP errors and user input parsing
Standardize error messages and date formatting across the app

app/utils/errors.ts

@@ -0,0 +1,3 @@
+export function getErrorMessage(error: any, fallback: string) {
+  return error?.data?.statusMessage || error?.message || fallback
+}