Refactor data fetching logic in SamplesContextProvider using separate useEffect hooks

### Context

The data fetching in `SamplesContextProvider` is currently handled by a single `useEffect` that triggers when either the `samplesQuery` or `myDataset` state changes. Within this `useEffect`, the `loading` state is set to `true` before making the API call.

### Problem or idea

We want to separate the data-fetching logic into multiple `useEffect` hooks:

- One `useEffect` for setting `loading` to `true` when `samplesQuery` or `myDataset` change.
- Another `useEffect` for making API call when `loading` changes to `true`.

### Solution or next step

Implement the two `usEffect` described above in `SamplesContextProvider`.