feat: server-side language preference — survives clear site data

Problem: language preference stored only in localStorage, lost on
'clear site data'. French users with browser lang=fr always got French
UI even when profile was English.

Solution:
- Backend: add 'language' field to user profile (default 'fr')
- Backend: PATCH /api/auth/me to update language preference
- Frontend: initI18n() priority: server > localStorage > navigator > 'fr'
- Frontend: initAuth() runs before initI18n() so server check works
- Frontend: profile Save persists language to server via PATCH

Verified: tabs show English after server profile set to 'en'

frontend/js/app.js

@@ -23,7 +23,12 @@ import { initMermaid, setupFocusMode } from './mermaid-viewer.js';
 // Initialization — mirrors the original app.js init() ordering
 // =========================================================================
 async function init() {
-  await initI18n();  // Must run before any rendering that uses t()
+  // 1. Init auth first — needed for server-side language preference
+  const authOk = await Auth.AuthManager.initAuth();
+
+  // 2. Init i18n after auth so server language preference is available
+  await initI18n();
+
   UI.initTheme();
   UI.initHeaderMenu();
   UI.initCustomDropdowns();
@@ -54,8 +59,6 @@ async function init() {
   initCommandPalette();
   initMobileToolbar();
 
-  const authOk = await Auth.AuthManager.initAuth();
-
   if (authOk) {
     Legacy.initSyncStatus();