feat: implement role-based access for settings route and add focus trap hook

- Wrapped the Settings component in RequireRole to enforce access control for admin and user roles.
- Introduced a new custom hook `useFocusTrap` to manage focus within modal dialogs, enhancing accessibility.
- Applied the focus trap in InviteModal, PermissionsModal, and UserDetailModal to prevent focus from leaving the dialog.
- Updated PassthroughLanding to focus on the heading when the component mounts.

backend/internal/api/middleware/auth.go

@@ -117,7 +117,7 @@ func RequireManagementAccess() gin.HandlerFunc {
 	return func(c *gin.Context) {
 		role := c.GetString("role")
 		if role == string(models.RolePassthrough) {
-			c.AbortWithStatusJSON(http.StatusForbidden, gin.H{"error": "Forbidden"})
+			c.AbortWithStatusJSON(http.StatusForbidden, gin.H{"error": "Pass-through users cannot access management features"})
 			return
 		}
 		c.Next()