fix(ui): sino de notificações não sobrepõe mais modais/telas

- remove z-[1002] do wrapper sempre-presente do sino (furava modais z-50..z-120)
- z aplicado só ao dropdown aberto (z-50) + backdrop (z-40)
- doc: RX apontado ao banco dev isolado

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/components/NotificationCenter.tsx

@@ -56,7 +56,7 @@ export const NotificationCenter: React.FC = () => {
     };
 
     return (
-        <div className="relative z-[1002]">
+        <div className="relative">
             <div className="relative">
                 <button
                     onClick={toggleOpen}
@@ -75,8 +75,8 @@ export const NotificationCenter: React.FC = () => {
 
                 {isOpen && (
                     <>
-                        <div className="fixed inset-0" onClick={toggleOpen} />
-                        <div className="absolute right-0 mt-4 w-[380px] bg-white/95 backdrop-blur-lg rounded-2xl shadow-[0_20px_50px_rgba(0,0,0,0.15)] border border-white/20 overflow-hidden animate-in fade-in zoom-in-95 slide-in-from-top-4 duration-300 transform origin-top-right">
+                        <div className="fixed inset-0 z-40" onClick={toggleOpen} />
+                        <div className="absolute right-0 mt-4 w-[380px] z-50 bg-white/95 backdrop-blur-lg rounded-2xl shadow-[0_20px_50px_rgba(0,0,0,0.15)] border border-white/20 overflow-hidden animate-in fade-in zoom-in-95 slide-in-from-top-4 duration-300 transform origin-top-right">
                             <div className="px-6 py-5 bg-gradient-to-r from-slate-50 to-white border-b border-slate-100 flex justify-between items-center">
                                 <div>
                                     <h3 className="font-bold text-slate-800 text-lg tracking-tight">Centro de Notificações</h3>