feat(credits): solde IA unique (option M), packs Stripe et polish billing

Un pot de crédits global (BASIC 100 / PRO 1 000 / BUSINESS 4 000) remplace
les seaux par fonction côté débit. Packs one-shot S/M/L, admin sans seaux
legacy, usage multi-features, hints de coût, anti-flash thème et hydratation
admin. Inclut aussi le pipeline slides renforcé et la page publique polishée.

memento-note/public/scripts/theme-init.js

@@ -21,18 +21,40 @@
       blue: 1,
     };
     var theme = allowed[raw] ? raw : 'light';
-    root.classList.remove('dark');
-    root.removeAttribute('data-theme');
+
+    // Synchroniser le cookie pour les prochains rendus serveur (anti-flash)
+    try {
+      document.cookie =
+        'memento-theme=' +
+        encodeURIComponent(theme) +
+        '; path=/; max-age=31536000; SameSite=Lax';
+      if (!stored) localStorage.setItem('theme-preference', theme);
+    } catch (e) {
+      /* ignore */
+    }
+
+    // Ne jamais retirer .dark avant de le remettre
+    var wantsDark = false;
     if (theme === 'auto') {
-      if (window.matchMedia('(prefers-color-scheme: dark)').matches) root.classList.add('dark');
-    } else if (theme === 'dark') {
+      wantsDark = window.matchMedia('(prefers-color-scheme: dark)').matches;
+    } else if (theme === 'dark' || theme === 'midnight') {
+      wantsDark = true;
+    }
+
+    if (wantsDark) {
       root.classList.add('dark');
-    } else if (theme === 'light') {
-      /* :root papier */
+    } else {
+      root.classList.remove('dark');
+    }
+
+    if (theme === 'auto' || theme === 'dark' || theme === 'light') {
+      root.removeAttribute('data-theme');
     } else {
       root.setAttribute('data-theme', theme);
-      if (theme === 'midnight') root.classList.add('dark');
     }
+
+    root.setAttribute('data-applied-theme', theme);
+
     var accentStored = localStorage.getItem('accent-color');
     var effectiveAccent = accentStored || defaultAccent;
     root.style.setProperty('--color-brand-accent', effectiveAccent);