Add stable CSS class hooks to Participation Summary table rows

[Copilot]

TallyResultsSummary lacked stable CSS selectors on its table rows, making it impossible for operators to reliably target individual participation metrics with custom CSS overrides.

Changes

• TallyResultsSummary.tsx

  • Added participation-summary-table class to the <Table> element.
  • Each <TableRow> now carries two classes: the shared participation-summary-row base class plus a row-specific semantic class (e.g. eligible-voters, total-valid-votes, blank-votes, weight).
  • Extracted repeated summaryRowSx and the summaryRowClassName helper to reduce duplication.

  // example rendered output
  <tr class="participation-summary-row total-valid-votes">…</tr>

• TallyResultsSummary.test.js — new Jest suite using renderToStaticMarkup that asserts every expected row class is present in the rendered markup.

• Docs — developers_admin-portal.md updated to document the new CSS hooks; release-next.md entry added.

Parent issue: https://github.com/sequentech/meta/issues/2608

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