fix: Sign out + login flow + cache bust for i18n

- auth.js: window.handleLogout at module top level (was inside
  renderUserMenu, not always called). Fixes settings dropdown
  'Sign out' onclick='handleLogout()' doing nothing.
- auth.js: add missing imports (loadVaults, loadTags from sidebar.js)
  and dynamic import for IndexUpdateManager (circular dep avoidance).
  Fixes 'loadVaults is not defined' error on login.
- sw.js: CACHE_VERSION v4→v5 to bust stale Service Worker cache
  that was serving old viewer.js without data-i18n attributes,
  causing dashboard tabs to stay French despite English profile.

To apply: rebuild container + unregister SW in DevTools + reload.

frontend/sw.js

@@ -1,6 +1,6 @@
 /* ObsiGate Service Worker - PWA v2 with IndexedDB offline support */
 
-const CACHE_VERSION = 'obsigate-v4-pwa';
+const CACHE_VERSION = 'obsigate-v5-pwa';
 const STATIC_CACHE = CACHE_VERSION + '-static';
 const DYNAMIC_CACHE = CACHE_VERSION + '-dynamic';
 const MAX_DYNAMIC_CACHE_SIZE = 100;