fix: user group pagination reset and model image caching (#19959)

- Changed default sort order in 'Edit Group' modal to 'created_at' (newest first).
  Previously it sorted by group membership, causing toggled users to invisible jump
  to the top of the list (often changing pages). using a stable sort fixes this UX issue.
- Removed unnecessary `page = 1` reset in toggleMember function so admins
  don't lose their place when selecting multiple users.

Bug 2 - Model Image Display Bug:
- Added `Cache-Control: no-cache, must-revalidate` headers to the model profile
  image endpoint. This ensures that when a model avatar is updated, the browser
  fetches the new image instead of serving a stale cached version (favicon).

Fixes #19885

src/lib/components/admin/Users/Groups/Users.svelte

@@ -30,7 +30,7 @@
 	let total = null;
 
 	let query = '';
-	let orderBy = `group_id:${groupId}`; // default sort key
+	let orderBy = 'created_at'; // default sort key
 	let direction = 'desc'; // default sort order
 
 	let page = 1;
@@ -42,6 +42,7 @@
 			orderBy = key;
 			direction = 'asc';
 		}
+		page = 1;
 	};
 
 	const getUserList = async () => {
@@ -75,7 +76,6 @@
 			});
 		}
 
-		page = 1;
 		getUserList();
 	};