feat: separate About/Clients/Partners into individual pages

- /[lang]/about/ — О компании (AboutSection standalone)
- /[lang]/clients/ — Нам доверяют (ClientsSection standalone)
- /[lang]/partners/ — Партнёры (PartnersSection standalone)
- Remove /o-kompanii/ page
- Update Header nav with all three links
- Add clients/partners keys to ru/en dictionaries
- sections: standalone prop → h1 heading, adjusted padding

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

src/components/layout/Header.tsx

@@ -14,7 +14,9 @@ export default function Header({ lang }: { lang: string }) {
 
   const links = [
     { label: d.nav.services, href: `/${lang}/uslugi/` },
-    { label: d.nav.about, href: `/${lang}/o-kompanii/` },
+    { label: d.nav.about, href: `/${lang}/about/` },
+    { label: d.nav.clients, href: `/${lang}/clients/` },
+    { label: d.nav.partners, href: `/${lang}/partners/` },
     { label: d.nav.faq, href: `/${lang}/faq/` },
   ]