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/i18n.js

@@ -91,12 +91,28 @@ export async function setLocale(locale) {
 }
 
 /** Initialise the i18n system. Call once, early, before any rendering.
- *  Reads saved preference from localStorage, falls back to navigator.language,
- *  then to 'fr'. */
+ *  Priority: server profile > localStorage > navigator.language > 'fr'. */
 export async function initI18n() {
-  // 1. Determine locale: saved > navigator.language > 'fr'
+  // 1. Determine locale: server profile > localStorage > navigator.language > 'fr'
   let locale = null;
-  try { locale = localStorage.getItem('obsigate-lang'); } catch (e) { /* */ }
+
+  // Try server-side language preference (survives clear site data)
+  try {
+    const resp = await fetch('/api/auth/me', { credentials: 'include' });
+    if (resp.ok) {
+      const user = await resp.json();
+      if (user.language === 'en' || user.language === 'fr') {
+        locale = user.language;
+      }
+    }
+  } catch (e) { /* not logged in or network error — fall through */ }
+
+  // Fall back to localStorage
+  if (!locale) {
+    try { locale = localStorage.getItem('obsigate-lang'); } catch (e) { /* */ }
+  }
+
+  // Fall back to navigator.language
   if (!locale) {
     const nav = (navigator.language || '').toLowerCase();
     locale = nav.startsWith('fr') ? 'fr'