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/app/page.tsx

@@ -5,6 +5,7 @@ import { Classes } from "@/components/sections/Classes";
 import { Pricing } from "@/components/sections/Pricing";
 import { FAQ } from "@/components/sections/FAQ";
 import { Contact } from "@/components/sections/Contact";
+import { BackToTop } from "@/components/ui/BackToTop";
 
 export default function HomePage() {
   return (
@@ -16,6 +17,7 @@ export default function HomePage() {
       <Pricing />
       <FAQ />
       <Contact />
+      <BackToTop />
     </>
   );
 }