[Grade C] Need to add loader to show the process of 'Clear all' function

### StatGPT Admin Frontend version

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### What is the problem this feature will solve?

1. Open any channel with Grade C dataset -> switch to Grade C dataset tab
2. Click on 'Clear all' button
3. Note that for several second there is the following view: the buttons are grey, the data is shown, no any explanation what is going on -> **issue**

### What is the feature you are proposing to solve the problem?

Let's have smth similar for deleting process as we've already implemented for indexing.
For example:
The loader:

The final result:

### What alternatives have you considered?

_No response_

Start by locating the existing indexing loader and the Grade C tab's “Clear all” handler. Reuse the relevant UI flow for the deletion process, then verify that a loader appears while clearing and that the requested final result is shown when the operation completes.

Written by the indexing model from the issue text.