feat: about stats cards, pricing popular badge, back-to-top button

- About: add 3 stat cards (13 trainers, 6 styles, 2 locations)
- Pricing: highlight first plan with gold "Популярный" badge
- BackToTop: floating gold button appears after 600px scroll

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

src/components/ui/BackToTop.tsx

@@ -0,0 +1,28 @@
+"use client";
+
+import { useState, useEffect } from "react";
+import { ChevronUp } from "lucide-react";
+
+export function BackToTop() {
+  const [visible, setVisible] = useState(false);
+
+  useEffect(() => {
+    function handleScroll() {
+      setVisible(window.scrollY > 600);
+    }
+    window.addEventListener("scroll", handleScroll, { passive: true });
+    return () => window.removeEventListener("scroll", handleScroll);
+  }, []);
+
+  return (
+    <button
+      onClick={() => window.scrollTo({ top: 0, behavior: "smooth" })}
+      aria-label="Наверх"
+      className={`fixed bottom-6 right-6 z-40 flex h-10 w-10 items-center justify-center rounded-full border border-[#c9a96e]/30 bg-black/60 text-[#d4b87a] backdrop-blur-sm transition-all duration-300 hover:bg-[#c9a96e]/20 hover:border-[#c9a96e]/50 ${
+        visible ? "translate-y-0 opacity-100" : "translate-y-4 opacity-0 pointer-events-none"
+      }`}
+    >
+      <ChevronUp size={18} />
+    </button>
+  );
+}