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>

frontend.css

@@ -736,28 +736,38 @@ body {
 ::-webkit-scrollbar-track { background: transparent; }
 ::-webkit-scrollbar-thumb { background: var(--border); border-radius: 3px; }
 
-/* ── Site Footer ──────────────────────────────────────────────── */
+/* ── Standings footer & branding ─────────────────────────────── */
 
-.site-footer {
-  flex-shrink: 0;
-  padding: 14px 20px;
+.standings__footer {
+  padding-top: 14px;
   border-top: 1px solid var(--border);
-  text-align: center;
-  font-size: 12px;
+  font-size: 11px;
   color: var(--text-muted);
-  line-height: 1.7;
+  line-height: 1.8;
+  margin-top: auto;
 }
 
-.site-footer__link {
+.standings__footer a {
   color: var(--text-dim);
   text-decoration: none;
 }
 
-.site-footer__link:hover {
+.standings__footer a:hover {
   color: var(--text);
   text-decoration: underline;
 }
 
+/* ── Player leaderboard name ─────────────────────────────────── */
+
+.lb-entry__name {
+  font-size: 12px;
+  font-weight: 500;
+  flex: 1;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
 /* ── Desktop (1024px+) ───────────────────────────────────────── */
 
 @media (min-width: 1024px) {
@@ -814,5 +824,4 @@ body {
     gap: 24px;
   }
 
-  .site-footer { display: none; }
 }