feat: add toUserMessage function for user-friendly error handling

frontends/nextjs/src/lib/errors/to-user-message.ts

@@ -0,0 +1,10 @@
+import { formatError } from './format-error'
+
+/**
+ * Convert error to user-friendly message (safe for display)
+ * Strips stack traces and internal details
+ */
+export function toUserMessage(error: unknown): string {
+  const formatted = formatError(error)
+  return formatted.message || 'An unexpected error occurred'
+}