feat: time-based credits, JUGADORES leaderboard, fix footer visibility

- Credits system: 1€/day, 5€/week, 15€/month time-based access via Redsys
  - credits table with token, tier, expires_at, status lifecycle
  - /api/credito/iniciar, /api/credito/estado, /api/pregunta/enviar endpoints
  - Polling-based token delivery to browser after Redsys URLOK redirect
  - localStorage token storage with expiry check on load
- JUGADORES leaderboard: top 7 players by questions used, polled every 60s
  - /api/jugadores endpoint, PlayerLeaderboard component in Standings sidebar
- Footer: moved into Standings sidebar (.standings__footer) so it's always visible
- pregunta.tsx: complete redesign with tier cards, credit badge, spinner, success state
- pregunta.css: new styles for tier cards, input, badge, spinner, success, link-btn

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

pregunta.css

@@ -173,6 +173,7 @@ body {
 .pregunta__links {
   display: flex;
   gap: 16px;
+  align-items: center;
   justify-content: center;
 }
 
@@ -185,3 +186,138 @@ body {
 .pregunta__links a:hover {
   color: var(--text-dim);
 }
+
+.pregunta__links-sep {
+  color: var(--text-muted);
+  font-size: 13px;
+}
+
+.pregunta__link-btn {
+  background: none;
+  border: none;
+  padding: 0;
+  color: var(--text-muted);
+  font-size: 13px;
+  font-family: var(--sans);
+  cursor: pointer;
+}
+
+.pregunta__link-btn:hover {
+  color: var(--text-dim);
+}
+
+/* ── Header row (logo + credit badge) ─────────────────────────── */
+
+.pregunta__header-row {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 12px;
+}
+
+.pregunta__credit-badge {
+  font-size: 12px;
+  font-weight: 600;
+  color: #4caf7d;
+  background: rgba(76, 175, 125, 0.12);
+  border: 1px solid rgba(76, 175, 125, 0.25);
+  border-radius: 20px;
+  padding: 4px 12px;
+  white-space: nowrap;
+}
+
+/* ── Success banner ────────────────────────────────────────────── */
+
+.pregunta__success {
+  background: rgba(76, 175, 125, 0.1);
+  border: 1px solid rgba(76, 175, 125, 0.25);
+  border-radius: 6px;
+  padding: 10px 14px;
+  color: #4caf7d;
+  font-size: 13px;
+  font-weight: 500;
+}
+
+/* ── Username input ────────────────────────────────────────────── */
+
+.pregunta__input {
+  width: 100%;
+  background: var(--surface);
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  padding: 12px;
+  color: var(--text);
+  font-family: var(--sans);
+  font-size: 14px;
+  line-height: 1.5;
+  transition: border-color 0.15s;
+}
+
+.pregunta__input:focus {
+  outline: none;
+  border-color: #444;
+}
+
+.pregunta__input::placeholder {
+  color: var(--text-muted);
+}
+
+/* ── Tier cards ────────────────────────────────────────────────── */
+
+.tier-cards {
+  display: grid;
+  grid-template-columns: repeat(3, 1fr);
+  gap: 10px;
+}
+
+.tier-card {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  gap: 6px;
+  padding: 18px 12px;
+  background: var(--surface);
+  border: 1px solid var(--border);
+  border-radius: 10px;
+  cursor: pointer;
+  font-family: var(--sans);
+  color: var(--text);
+  transition: border-color 0.15s, background 0.15s;
+}
+
+.tier-card:hover {
+  border-color: #444;
+}
+
+.tier-card--selected {
+  border-color: var(--accent);
+  background: rgba(217, 119, 87, 0.08);
+}
+
+.tier-card__price {
+  font-size: 22px;
+  font-weight: 700;
+  color: var(--accent);
+}
+
+.tier-card__label {
+  font-size: 12px;
+  color: var(--text-dim);
+}
+
+/* ── Spinner ───────────────────────────────────────────────────── */
+
+.pregunta__spinner {
+  width: 32px;
+  height: 32px;
+  border: 3px solid var(--border);
+  border-top-color: var(--accent);
+  border-radius: 50%;
+  animation: pregunta-spin 0.8s linear infinite;
+  margin: 8px auto;
+}
+
+@keyframes pregunta-spin {
+  to { transform: rotate(360deg); }
+}