feat: remove formation/pricing page and all related content

Formation was a SaaS pricing page unrelated to the portfolio.
Removed: page, nav link, locale keys (nav.formation, seo.formation,
pricing.*) in both FR and EN, legacy source files.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/components/layout/AppHeader.vue

@@ -11,7 +11,6 @@ const navLinks = computed(() => [
   { key: 'about', path: '/about' },
   { key: 'contact', path: '/contact' },
   { key: 'fiverr', path: '/fiverr' },
-  { key: 'formation', path: '/formation' },
 ])
 
 function toggleLocale() {