statistics: replace $.ajax with lorisFetch

[Montekkundan]

Summary

Replace $.ajax calls with lorisFetch in statistics.

Why

This removes jQuery AJAX usage in this module for issue #4213.

Scope

Only AJAX replacement in this module. No unrelated jQuery refactors.

Dependency

Depends on PR #10333 .

Verification

git grep '\$\.ajax' is clean for touched files.

[Montekkundan]

reviewed this pr against the new Client guidance and did not push code changes.

• The touched calls in this PR are HTML fragment/tab content requests (response.text() + DOM injection), not REST-style JSON data requests.
• Per guidance, those flows remain on lorisFetch.
• I can follow up with a separate Client migration if/when JSON endpoints are introduced for these paths.