feat: question-count tiers, footer in main, pregunta footer

- Pricing: 0,99€/10q · 9,99€/200q · 19,99€/unlimited (all 30 days)
- DB: max_questions + questions_used columns on credits table;
  consumeCreditQuestion() atomically validates, creates question,
  and decrements quota in a single transaction
- Server: updated CREDIT_TIERS, /api/credito/estado returns questionsLeft,
  /api/pregunta/enviar returns updated questionsLeft after each submission
- pregunta.tsx: badge shows live question count; submit disabled when exhausted;
  questionsLeft synced to localStorage after each submission; Cloud Host footer added
- footer: moved from Standings sidebar into <main> (scrolls with game content);
  also added to all pregunta page states

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

frontend.css

@@ -736,23 +736,24 @@ body {
 ::-webkit-scrollbar-track { background: transparent; }
 ::-webkit-scrollbar-thumb { background: var(--border); border-radius: 3px; }
 
-/* ── Standings footer & branding ─────────────────────────────── */
+/* ── Site footer (inside main) ───────────────────────────────── */
 
-.standings__footer {
-  padding-top: 14px;
+.site-footer {
+  flex-shrink: 0;
+  margin-top: 32px;
+  padding: 16px 0 4px;
   border-top: 1px solid var(--border);
   font-size: 11px;
   color: var(--text-muted);
-  line-height: 1.8;
-  margin-top: auto;
+  line-height: 1.9;
 }
 
-.standings__footer a {
+.site-footer a {
   color: var(--text-dim);
   text-decoration: none;
 }
 
-.standings__footer a:hover {
+.site-footer a:hover {
   color: var(--text);
   text-decoration: underline;
 }