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>

game.ts

@@ -68,6 +68,7 @@ export type RoundState = {
   viewerVotesB?: number;
   viewerVotingEndsAt?: number;
   userAnswers?: { username: string; text: string }[];
+  userAnswerVotes?: Record<string, number>;
 };
 
 export type GameState = {
@@ -266,7 +267,7 @@ export async function callVote(
   return cleaned.startsWith("A") ? "A" : "B";
 }
 
-import { saveRound, getNextPendingQuestion, markQuestionUsed } from "./db.ts";
+import { saveRound, getNextPendingQuestion, markQuestionUsed, persistUserAnswerVotes } from "./db.ts";
 
 // ── Game loop ───────────────────────────────────────────────────────────────
 
@@ -498,6 +499,11 @@ export async function runGame(
       continue;
     }
 
+    // Persist votes for user answers
+    if (round.userAnswerVotes && round.userAnswers && round.userAnswers.length > 0) {
+      persistUserAnswerVotes(round.num, round.userAnswerVotes);
+    }
+
     // Archive round
     saveRound(round);
     state.completed = [...state.completed, round];