feat: i18n system - French/English internationalization

Core: i18n.js module with t(), setLocale(), initI18n()
- Flat key→string JSON locale files (fr.json + en.json, 165 keys)
- data-i18n / data-i18n-placeholder / data-i18n-attr DOM attributes
- Live locale switch via setLocale() - no page reload
- Language detection: localStorage > navigator.language > 'fr'
- Fallback: missing key → French → raw key

Wired into:
- app.js: await initI18n() early in init()
- config.js: profile save → await setLocale(), toasts + confirm via t()
- auth.js: network error toast via t()
- index.html: data-i18n on login form, profile section, search placeholder

frontend/js/config.js

@@ -5,6 +5,7 @@ import { el, icon, openFile } from './viewer.js';
 import { syncVaultSelectors, setSelectedVaultContext, refreshSidebarForContext, loadVaults, loadVaultSettings, loadTags, TagFilterService, refreshSidebarTreePreservingState } from './sidebar.js';
 import {  escapeHtml, safeCreateIcons } from './utils.js';
 import { showToast, closeHeaderMenu, closeMobileSidebar } from './ui.js';
+import { t, setLocale, getLocale } from './i18n.js';
 
 let _recentTimestampTimer = null;
 let _recentFilesCache = [];
@@ -769,7 +770,7 @@ function saveFrontendConfig() {
     search_timeout_ms: _getFieldNum("cfg-timeout", 30000),
   };
   localStorage.setItem(_FRONTEND_CONFIG_KEY, JSON.stringify(cfg));
-  showToast("Paramètres client sauvegardés", "success");
+  showToast(t("toast.config_client_saved"), "success");
 }
 
 async function saveBackendConfig() {
@@ -793,7 +794,7 @@ async function saveBackendConfig() {
       body: JSON.stringify(body),
     });
     if (res.ok) {
-      showToast("Configuration backend sauvegardée", "success");
+      showToast(t("toast.config_saved"), "success");
     } else {
       const errorData = await res.json().catch(() => ({}));
       showToast(errorData.detail || "Erreur de sauvegarde", "error");
@@ -812,7 +813,7 @@ async function forceReindex() {
   }
   try {
     await api("/api/index/reload");
-    showToast("Réindexation terminée", "success");
+    showToast(t("toast.reindex_done"), "success");
     loadDiagnostics();
     await Promise.all([loadVaults(), loadTags()]);
   } catch (err) {
@@ -853,7 +854,7 @@ async function resetConfigDefaults() {
     console.error("Reset config error:", err);
   }
   loadConfigFields();
-  showToast("Configuration réinitialisée", "success");
+  showToast(t("toast.config_reset"), "success");
 }
 
 async function loadDiagnostics() {
@@ -1549,15 +1550,16 @@ function initProfile() {
 
   // Load saved profile
   try {
-    var lang = localStorage.getItem('obsigate-lang') || 'fr';
+    var lang = getLocale();
     var name = localStorage.getItem('obsigate-name') || '';
     if (langSel) langSel.value = lang;
     if (nameInp) nameInp.value = name;
   } catch(e) {}
 
-  saveBtn.addEventListener('click', function() {
+  saveBtn.addEventListener('click', async function() {
     try {
-      localStorage.setItem('obsigate-lang', langSel.value);
+      var newLang = langSel.value;
+      await setLocale(newLang);   // Live-reload all translations
       if (nameInp) localStorage.setItem('obsigate-name', nameInp.value.trim());
       if (savedEl) { savedEl.style.display = 'inline'; setTimeout(function() { savedEl.style.display = 'none'; }, 2000); }
     } catch(e) {}
@@ -1566,7 +1568,7 @@ function initProfile() {
   // Logout button
   if (logoutBtn) {
     logoutBtn.addEventListener('click', function() {
-      if (confirm('Voulez-vous vraiment vous deconnecter ?')) {
+      if (confirm(t('confirm.logout'))) {
         fetch('/api/auth/logout', { method: 'POST' })
           .then(function() { window.location.href = '/'; })
           .catch(function() { window.location.href = '/'; });