feat: add complex user invitation and management features

- Implemented complex member and invitation schemas using Zod for validation.
- Created new API endpoints for inviting, accepting, revoking, and canceling complex user invitations.
- Developed handlers for managing complex users, including listing, inviting, revoking, and canceling invitations.
- Added frontend components for displaying and managing complex users and invitations.
- Introduced session storage management for pending invitations.
- Integrated Gravatar for user avatars based on email.
- Created database migration for complex invitations table.

apps/frontend/src/components/user-avatar.tsx

@@ -0,0 +1,47 @@
+import { Avatar, AvatarFallback, AvatarImage } from '@/components/ui/avatar';
+import { useEffect, useState } from 'react';
+
+type UserAvatarProps = {
+  src?: string | null;
+  alt: string;
+  fallbackText: string;
+  size?: 'default' | 'sm' | 'lg';
+  className?: string;
+};
+
+function getInitials(text: string) {
+  const words = text.trim().split(/\s+/).filter(Boolean).slice(0, 2);
+  const initials = words.map((word) => word[0]?.toUpperCase() ?? '').join('');
+  return initials || 'U';
+}
+
+export function UserAvatar({
+  src,
+  alt,
+  fallbackText,
+  size = 'default',
+  className,
+}: UserAvatarProps) {
+  const [imageError, setImageError] = useState(false);
+
+  useEffect(() => {
+    setImageError(false);
+  }, [src]);
+
+  const canShowImage = Boolean(src) && !imageError;
+
+  return (
+    <Avatar size={size} className={className}>
+      {canShowImage && (
+        <AvatarImage
+          src={src ?? undefined}
+          alt={alt}
+          onError={() => {
+            setImageError(true);
+          }}
+        />
+      )}
+      <AvatarFallback>{getInitials(fallbackText)}</AvatarFallback>
+    </Avatar>
+  );
+}