style(app): mobile-first layout polish

src/components/FloatingButton.tsx

@@ -9,7 +9,7 @@ function FloatingButton({ onClick, label = 'Créer une note' }: FloatingButtonPr
       type="button"
       onClick={onClick}
       aria-label={label}
-      className="fixed bottom-6 right-6 w-14 h-14 rounded-full bg-amber-400 text-3xl text-gray-800 shadow-lg active:scale-95"
+      className="fixed bottom-6 right-6 z-20 flex h-14 w-14 items-center justify-center rounded-full bg-amber-400 text-3xl text-gray-800 shadow-lg transition-transform hover:bg-amber-500 active:scale-95 focus:outline-none focus:ring-2 focus:ring-amber-600 focus:ring-offset-2"
     >
       +
     </button>