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/pages/formation.vue

@@ -1,21 +0,0 @@
-<script setup lang="ts">
-const { t } = useI18n()
-
-useSeoMeta({
-  title: () => t('seo.formation.title'),
-  description: () => t('seo.formation.description'),
-  ogTitle: () => t('seo.formation.title'),
-  ogDescription: () => t('seo.formation.description'),
-  ogImage: 'https://killiandalcin.fr/og-image.png',
-  ogImageWidth: 1200,
-  ogImageHeight: 630,
-  ogType: 'website',
-})
-</script>
-
-<template>
-  <div class="max-w-7xl mx-auto px-4 sm:px-6 lg:px-8 py-12">
-    <h1 class="text-2xl font-bold">{{ t('nav.formation') }}</h1>
-    <p class="text-gray-600 dark:text-gray-400 mt-4">Phase 3 content placeholder</p>
-  </div>
-</template>