feature: a minimal implementation of the appointment feature done.

frontend/src/lib/formatApiError.ts

@@ -0,0 +1,12 @@
+export function formatApiErrorMessage(err: unknown, fallback: string): string {
+  if (err && typeof err === 'object' && 'message' in err) {
+    const m = (err as { message: unknown }).message;
+    if (Array.isArray(m)) {
+      return m.filter(Boolean).join(', ');
+    }
+    if (typeof m === 'string' && m.trim()) {
+      return m;
+    }
+  }
+  return fallback;
+}