feat: add viewer voting on user answers with leaderboard scoring

Viewers can now vote for their favourite audience answers during the
30-second voting window. Votes are persisted to the DB at round end
and aggregated as SUM(votes) in the JUGADORES leaderboard.

- db.ts: add persistUserAnswerVotes(); switch getPlayerScores() to SUM(votes)
- game.ts: add userAnswerVotes to RoundState; persist votes before saveRound
- server.ts: add userAnswerVoters map + /api/vote/respuesta endpoint
- frontend.tsx: add userAnswerVotes type; vote state/handler in App; ▲ buttons in Arena
- frontend.css: flex layout for user-answer rows; user-vote-btn styles

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend.css

@@ -796,6 +796,53 @@ body {
 .user-answer {
   font-size: 13px;
   line-height: 1.4;
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  justify-content: space-between;
+}
+
+.user-answer__main {
+  flex: 1;
+  min-width: 0;
+}
+
+.user-answer__vote {
+  display: flex;
+  align-items: center;
+  gap: 5px;
+  flex-shrink: 0;
+}
+
+.user-vote-btn {
+  background: none;
+  border: 1px solid var(--border);
+  border-radius: 4px;
+  color: var(--text-muted);
+  font-size: 10px;
+  padding: 2px 7px;
+  cursor: pointer;
+  transition: border-color 0.15s, color 0.15s;
+  line-height: 1.4;
+}
+
+.user-vote-btn:hover {
+  border-color: #444;
+  color: var(--text-dim);
+}
+
+.user-vote-btn--active {
+  border-color: var(--accent);
+  color: var(--accent);
+}
+
+.user-vote-count {
+  font-family: var(--mono);
+  font-size: 11px;
+  font-weight: 700;
+  color: var(--text-muted);
+  min-width: 14px;
+  text-align: right;
 }
 
 .user-answer__name {