feat(elections): results UI with round-by-round visualization

Summary

Display tabulation results to admins (preview) and to the public (after publish) with a round-by-round breakdown that makes ranked-choice results legible.

Part of #1933.

Requirements

• Admin-only results preview once election status is `closed`
• Public results page once status is `published`
• Per-race display: method (IRV/STV), winner(s), seats filled, quota (for STV)
• Round-by-round table showing counts, eliminations, transfers
• Visualization (stacked bar chart or Sankey) showing ballot flow across rounds
• Clear labeling of exhausted ballots in every round
• Shareable per-election URL
• Downloadable CSV of per-round counts for each race
• No raw ballots exposed in the UI — only aggregate round data

Context

• Epic: #1933
• Depends on IRV and STV tabulation issues

Implementation notes

• Sankey visualization library: `d3-sankey` directly, or `@nivo/sankey` if we want a pre-built React component
• Use design-system color tokens — results coloring must meet contrast requirements