Feat: intégration multilingue FR/EN (i18n)

- Ajout de src/i18n/index.js : dictionnaire centralisé + fonction t(key, vars)
- Ajout de LanguageSwitcher.svelte : toggle FR/EN avec persistance localStorage
- Router : normalizePath strip /en/, apiPrefix() pour les fetches, détection langue (URL > localStorage > navigator)
- Tous les composants (Header, Menu, Footer, Article, Blog, Play) migrent vers t() depuis @i18n
- Blog : navigation interne (fetch, history, getSlugFromUrl) locale-aware

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

src/views/Article.svelte

@@ -4,6 +4,7 @@
    * Reçoit les données article via props, pas via le slide system.
    */
   import Footer from '@components/layout/Footer.svelte'
+  import { t } from '@i18n'
 
   let { data, onBack } = $props()
 
@@ -25,27 +26,27 @@
 
   <!-- Date + share buttons -->
   <div class="article-topbar">
-    <time class="article-date">Publié le {data.published}</time>
+    <time class="article-date">{t('published_on')} {data.published}</time>
     <div class="article-share">
       {#if copySuccess}
-        <span class="copy-toast">Lien copié !</span>
+        <span class="copy-toast">{t('link_copied')}</span>
       {/if}
       <div class="share-buttons">
         <!-- svelte-ignore a11y_invalid_attribute -->
-        <a href="#" class="share-button" title="Copier le lien" onclick={(e) => { e.preventDefault(); copyLink() }}>
-          <img src="/assets/icons/link-icon.svg" alt="Copier le lien" width="20" height="20" />
+        <a href="#" class="share-button" title={t('copy_link')} onclick={(e) => { e.preventDefault(); copyLink() }}>
+          <img src="/assets/icons/link-icon.svg" alt={t('copy_link')} width="20" height="20" />
         </a>
         <a href="https://api.whatsapp.com/send?text={shareUrl}" target="_blank" rel="noopener noreferrer" class="share-button">
-          <img src="/assets/icons/whatsapp-icon.svg" alt="Partager sur WhatsApp" width="20" height="20" />
+          <img src="/assets/icons/whatsapp-icon.svg" alt={t('share_whatsapp')} width="20" height="20" />
         </a>
         <a href="https://twitter.com/intent/tweet?url={shareUrl}" target="_blank" rel="noopener noreferrer" class="share-button">
-          <img src="/assets/icons/twitter-icon.svg" alt="Partager sur X" width="20" height="20" />
+          <img src="/assets/icons/twitter-icon.svg" alt={t('share_x')} width="20" height="20" />
         </a>
         <a href="https://www.facebook.com/sharer/sharer.php?u={shareUrl}" target="_blank" rel="noopener noreferrer" class="share-button">
-          <img src="/assets/icons/facebook-icon.svg" alt="Partager sur Facebook" width="20" height="20" />
+          <img src="/assets/icons/facebook-icon.svg" alt={t('share_facebook')} width="20" height="20" />
         </a>
         <a href="https://www.linkedin.com/sharing/share-offsite/?url={shareUrl}" target="_blank" rel="noopener noreferrer" class="share-button">
-          <img src="/assets/icons/linkedin-icon.svg" alt="Partager sur LinkedIn" width="20" height="20" />
+          <img src="/assets/icons/linkedin-icon.svg" alt={t('share_linkedin')} width="20" height="20" />
         </a>
       </div>
     </div>
@@ -74,23 +75,23 @@
   <!-- Share section (bas d'article) -->
   <div class="article-share-section">
     <hr class="share-divider" />
-    <p class="share-label">Partager cet article</p>
+    <p class="share-label">{t('share_article')}</p>
     <div class="share-buttons">
       <!-- svelte-ignore a11y_invalid_attribute -->
-      <a href="#" class="share-button" title="Copier le lien" onclick={(e) => { e.preventDefault(); copyLink() }}>
-        <img src="/assets/icons/link-icon.svg" alt="Copier le lien" width="20" height="20" />
+      <a href="#" class="share-button" title={t('copy_link')} onclick={(e) => { e.preventDefault(); copyLink() }}>
+        <img src="/assets/icons/link-icon.svg" alt={t('copy_link')} width="20" height="20" />
       </a>
       <a href="https://api.whatsapp.com/send?text={shareUrl}" target="_blank" rel="noopener noreferrer" class="share-button">
-        <img src="/assets/icons/whatsapp-icon.svg" alt="Partager sur WhatsApp" width="20" height="20" />
+        <img src="/assets/icons/whatsapp-icon.svg" alt={t('share_whatsapp')} width="20" height="20" />
       </a>
       <a href="https://twitter.com/intent/tweet?url={shareUrl}" target="_blank" rel="noopener noreferrer" class="share-button">
-        <img src="/assets/icons/twitter-icon.svg" alt="Partager sur X" width="20" height="20" />
+        <img src="/assets/icons/twitter-icon.svg" alt={t('share_x')} width="20" height="20" />
       </a>
       <a href="https://www.facebook.com/sharer/sharer.php?u={shareUrl}" target="_blank" rel="noopener noreferrer" class="share-button">
-        <img src="/assets/icons/facebook-icon.svg" alt="Partager sur Facebook" width="20" height="20" />
+        <img src="/assets/icons/facebook-icon.svg" alt={t('share_facebook')} width="20" height="20" />
       </a>
       <a href="https://www.linkedin.com/sharing/share-offsite/?url={shareUrl}" target="_blank" rel="noopener noreferrer" class="share-button">
-        <img src="/assets/icons/linkedin-icon.svg" alt="Partager sur LinkedIn" width="20" height="20" />
+        <img src="/assets/icons/linkedin-icon.svg" alt={t('share_linkedin')} width="20" height="20" />
       </a>
     </div>
   </div>
@@ -98,7 +99,7 @@
   <!-- Articles recommandés -->
   {#if data.related?.length}
     <section class="article-related">
-      <h2>Nos recommandations</h2>
+      <h2>{t('related')}</h2>
       <div class="article-related-grid">
         {#each data.related as rec}
           <a href="/blog/{rec.slug}" class="article-related-card">