Media: Add selection count display for bulk select mode in the Media Library

[AKSHAT2802]

Trac ticket: https://core.trac.wordpress.org/ticket/63926

This PR introduces a live selection count label that appears in the toolbar whenever Bulk Select mode is active, showing "X item(s) selected" and updating in real time as items are checked or unchecked.
The count is hidden when Bulk Select mode is not active, so it has no visual impact on the normal Media Library view.

How to test

1. Checkout to branch and build files
2. Navigate to Media Library (/wp-admin/upload.php) in grid view.
3. Click Bulk Select.
4. Select several images — the toolbar should display "X items selected", updating with each click.
5. Click Cancel — the count should disappear, and the toolbar should return to its normal state.

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