refactor(data-table): keep ColumnSettings composable (toolbar API consistency)

[itsprade]

Draft — API consistency follow-up to #382. The columns feature shipped in #382 (now on main, not yet released). This is a small refactor of the toolbar API before release, per review feedback.

Problem

#382 merged with column settings exposed as a boolean prop on the toolbar:

<DataTable.Toolbar columnSettings>
  <DataTable.Filters />
</DataTable.Toolbar>

That's inconsistent with every other toolbar piece (DataTable.Filters, DataTable.Pagination), which are composed sub-components. Flagged by @seiya — worth cleaning up before the next release so the API is uniform.

Change

Revert to a composed sub-component, DataTable.ColumnSettings:

<DataTable.Toolbar>
  <DataTable.Filters />
  <DataTable.ColumnSettings />   {/* right-aligns itself */}
</DataTable.Toolbar>

• DataTable.ColumnSettings is public again (matches Filters / Pagination).
• It carries ml-auto, so filters flow from the left and the Columns control parks on the right — one row, no consumer layout code.
• DataTable.Toolbar is a plain flex row (drops the columnSettings prop).

Behavior and appearance are unchanged — this is purely an API shape change (and the feature hasn't shipped yet, so it's not a breaking change).

Not included (separate follow-up)

@seiya's larger idea — splitting Filters into slots (slot="add" addIconOnly / slot="chips") for finer toolbar-layout control — is a bigger Filters refactor and is not in this PR. Proposed as a follow-up so we can land the consistency fix now.

Verification

type-check, lint, test (1394 pass), oxfmt --check, and examples/vite-app type-check all pass; verified live in the DataTable Lab (filters + Columns share one row, popover works).

🤖 Generated with Claude Code

[IzumiSy]

@copilot rebase branch to main

[Copilot]

@copilot rebase branch to main

Rebased locally onto main; the updated HEAD is 962d9da. Pushing the rebased branch back to the PR is currently blocked by an engine-tools-report_progress GPG signing failure in the sync step.