Center the leaderboard rank column

[StevenDillmann]

The rank column's header and values were right-aligned (text-right), which
pushed RANK and its numbers against the neighbouring Agent column while
leaving a gap on the left of a fixed w-12 column.

Switches both the header and cell alignment to text-center so the label sits
over its own values and the column reads as a self-contained gutter.

Two class changes in buildColumns, components/leaderboard/leaderboard-table.tsx:

-      headerClassName: 'w-12 text-right',
-      cellClassName: 'text-right',
+      headerClassName: 'w-12 text-center',
+      cellClassName: 'text-center',

These are consumed by components/ui/data-table.tsx (meta?.headerClassName
at the header cell, meta?.cellClassName at the body cell), so no other
call sites are affected. npm run types:check passes.

🤖 Generated with Claude Code

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