Provider list: align page 1 row count, make top-group fold both ways

[koenmtb1]

Two fixes to the providers table:

• Page 1 was showing only 5 individual providers below the "Top 5" group row (because pagination still served 10 raw providers, the first 5 of which got collapsed). Pagination now serves 14 on page 1, the first 5 collapse into the group row and 9 render individually, so the visible row count matches every other page (10 slots).
• The "Top 5" group row could be expanded but not collapsed back. The row now stays rendered in both states, click toggles, the chevron rotates, and the top-5 individual rows appear below it with the decentralisation bar between top and rest.

TOP_GROUP_SIZE is now exported from useProviderTable and reused by useProviderTableDisplayData so the pagination math and the display flag can't drift.