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

backend/open_webui/routers/models.py

@@ -291,12 +291,15 @@ async def get_model_by_id(id: str, user=Depends(get_verified_user)):
 @router.get("/model/profile/image")
 async def get_model_profile_image(id: str, user=Depends(get_verified_user)):
     model = Models.get_model_by_id(id)
+    # Cache-control headers to prevent stale cached images
+    cache_headers = {"Cache-Control": "no-cache, must-revalidate"}
+
     if model:
         if model.meta.profile_image_url:
             if model.meta.profile_image_url.startswith("http"):
                 return Response(
                     status_code=status.HTTP_302_FOUND,
-                    headers={"Location": model.meta.profile_image_url},
+                    headers={"Location": model.meta.profile_image_url, **cache_headers},
                 )
             elif model.meta.profile_image_url.startswith("data:image"):
                 try:
@@ -307,14 +310,17 @@ async def get_model_profile_image(id: str, user=Depends(get_verified_user)):
                     return StreamingResponse(
                         image_buffer,
                         media_type="image/png",
-                        headers={"Content-Disposition": "inline; filename=image.png"},
+                        headers={
+                            "Content-Disposition": "inline; filename=image.png",
+                            **cache_headers,
+                        },
                     )
                 except Exception as e:
                     pass
 
-        return FileResponse(f"{STATIC_DIR}/favicon.png")
+        return FileResponse(f"{STATIC_DIR}/favicon.png", headers=cache_headers)
     else:
-        return FileResponse(f"{STATIC_DIR}/favicon.png")
+        return FileResponse(f"{STATIC_DIR}/favicon.png", headers=cache_headers)
 
 
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