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/router/index.js

@@ -5,7 +5,12 @@ import { locale } from "@state/locale.svelte";
 let siteInitialized = false;
 
 function normalizePath(path) {
-  return path === "/" ? "/home" : path;
+  const stripped = path.replace(/^\/en(\/|$)/, '$1') || '/';
+  return stripped === '/' ? '/home' : stripped;
+}
+
+function apiPrefix() {
+  return locale.current === 'en' ? '/en' : '';
 }
 
 /**
@@ -47,8 +52,7 @@ async function loadSlide(path) {
   }
 
   try {
-    // Fetch the actual slide path (parent), not the sub-page
-    const response = await fetch(`${slidePath}.json`);
+    const response = await fetch(`${apiPrefix()}${slidePath}.json`);
     if (!response.ok) throw new Error(`HTTP ${response.status}`);
     const data = await response.json();
 
@@ -76,7 +80,10 @@ export function slideTo(path, { skipHistory = false } = {}) {
   path = normalizePath(path);
 
   if (!skipHistory) {
-    history.pushState({}, "", path === "/home" ? "/" : path);
+    const historyPath = locale.current === 'en'
+      ? (path === '/home' ? '/en' : `/en${path}`)
+      : (path === '/home' ? '/' : path);
+    history.pushState({}, '', historyPath);
   }
 
   const idx = findSlideIndex(path);
@@ -94,6 +101,22 @@ export function slideTo(path, { skipHistory = false } = {}) {
 }
 
 export async function initRouter() {
+  // Language detection: URL prefix > localStorage > navigator
+  const hasEnPrefix = window.location.pathname.startsWith('/en');
+  if (hasEnPrefix) {
+    locale.setLanguage('en');
+    localStorage.setItem('wg_lang', 'en');
+  } else if (!localStorage.getItem('wg_lang')) {
+    const navLang = navigator.language || navigator.languages?.[0] || 'fr';
+    if (navLang.startsWith('en')) {
+      window.location.replace('/en' + window.location.pathname);
+      return;
+    }
+  } else if (localStorage.getItem('wg_lang') === 'en') {
+    window.location.replace('/en' + window.location.pathname);
+    return;
+  }
+
   const initialPath = normalizePath(window.location.pathname);
 
   await loadSlide(initialPath);