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/locales/en.ts

@@ -2,6 +2,8 @@ export const en = {
   nav: {
     services: 'Services',
     about: 'About',
+    clients: 'Clients',
+    partners: 'Partners',
     faq: 'FAQ',
     contact: 'Contact',
   },