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/config.js

@@ -1561,6 +1561,17 @@ function initProfile() {
       var newLang = langSel.value;
       await setLocale(newLang);   // Live-reload all translations
       if (nameInp) localStorage.setItem('obsigate-name', nameInp.value.trim());
+
+      // Persist language to server (survives clear site data)
+      try {
+        await fetch('/api/auth/me', {
+          method: 'PATCH',
+          headers: { 'Content-Type': 'application/json' },
+          credentials: 'include',
+          body: JSON.stringify({ language: newLang }),
+        });
+      } catch (e) { /* non-bloquant */ }
+
       if (savedEl) { savedEl.style.display = 'inline'; setTimeout(function() { savedEl.style.display = 'none'; }, 2000); }
     } catch(e) {}
   });