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>

db.ts

@@ -95,16 +95,28 @@ export function markQuestionUsed(id: number): void {
   db.prepare("UPDATE questions SET status = 'used' WHERE id = $id").run({ $id: id });
 }
 
-/** Top 7 players by number of answers submitted, excluding anonymous. */
+/** Top 7 players by total votes received on their answers, excluding anonymous. */
 export function getPlayerScores(): Record<string, number> {
   const rows = db
     .query(
-      "SELECT username, COUNT(*) as score FROM user_answers WHERE username != '' GROUP BY username ORDER BY score DESC LIMIT 7"
+      "SELECT username, SUM(votes) as score FROM user_answers WHERE username != '' GROUP BY username ORDER BY score DESC LIMIT 7"
     )
     .all() as { username: string; score: number }[];
   return Object.fromEntries(rows.map(r => [r.username, r.score]));
 }
 
+/** Persist accumulated vote counts for user answers in a given round. */
+export function persistUserAnswerVotes(roundNum: number, votes: Record<string, number>): void {
+  const stmt = db.prepare(
+    "UPDATE user_answers SET votes = $votes WHERE round_num = $roundNum AND username = $username"
+  );
+  db.transaction(() => {
+    for (const [username, voteCount] of Object.entries(votes)) {
+      stmt.run({ $votes: voteCount, $roundNum: roundNum, $username: username });
+    }
+  })();
+}
+
 // ── User answers (submitted during live rounds) ──────────────────────────────
 
 db.exec(`
@@ -114,10 +126,18 @@ db.exec(`
     text TEXT NOT NULL,
     username TEXT NOT NULL,
     token TEXT NOT NULL,
+    votes INTEGER NOT NULL DEFAULT 0,
     created_at DATETIME DEFAULT CURRENT_TIMESTAMP
   );
 `);
 
+// Migration: add votes column to pre-existing user_answers tables
+try {
+  db.exec("ALTER TABLE user_answers ADD COLUMN votes INTEGER NOT NULL DEFAULT 0");
+} catch {
+  // Column already exists — no-op
+}
+
 // ── Credits (answer-count-based access) ──────────────────────────────────────
 
 db.exec(`