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/formatters.ts

@@ -0,0 +1,10 @@
+export function formatDateTime(value: string | null, fallback = 'Not available') {
+  if (!value) {
+    return fallback
+  }
+
+  return new Intl.DateTimeFormat('en-MY', {
+    dateStyle: 'medium',
+    timeStyle: 'short'
+  }).format(new Date(value))
+}