fix: re-sync language from server after login

Root cause: initI18n() runs before auth on login page, so
/api/auth/me fails (401) and locale falls back to browser language.
After login, showWelcome() renders with stale locale without
re-checking server preference. French users always got French UI.

Fix: after successful login, fetch /api/auth/me and call setLocale()
if server language differs from current. This ensures the dashboard
renders in the user's saved language preference.

frontend/js/auth.js

@@ -294,6 +294,19 @@ function initLoginForm() {
     try {
       await AuthManager.login(username, password, rememberMe);
       AuthManager.showApp();
+
+      // Re-sync language from server now that we're authenticated
+      try {
+        const resp = await fetch('/api/auth/me', { credentials: 'include' });
+        if (resp.ok) {
+          const user = await resp.json();
+          if (user.language && user.language !== getLocale()) {
+            const { setLocale } = await import('./i18n.js');
+            await setLocale(user.language);
+          }
+        }
+      } catch (e) { /* non-bloquant */ }
+
       // Load app data after successful login
       try {
         await Promise.all([loadVaults(), loadTags()]);