fix: SSE sync indicator stuck on 'Connexion...' (3 fixes)

1. Move initSyncStatus() AFTER auth check — EventSource was connecting
   before the access_token cookie was available, causing 401 errors.

2. Reconnect SSE after login — Login form handler now calls
   IndexUpdateManager.connect() + showWelcome() after successful auth.

3. SSESafeGZipMiddleware — GZip buffering breaks Server-Sent Events
   streaming. Custom middleware subclass skips compression for
   /api/events endpoint (path-based bypass).

frontend/app.js

@@ -1734,6 +1734,10 @@
         // Load app data after successful login
         try {
           await Promise.all([loadVaults(), loadTags()]);
+          // Start SSE sync now that auth cookie is set
+          IndexUpdateManager.connect();
+          // Show dashboard
+          showWelcome();
         } catch (err) {
           console.error("Failed to load data after login:", err);
         }
@@ -7818,7 +7822,6 @@
     initSidebarFilter();
     initSidebarResize();
     initEditor();
-    initSyncStatus();
     initLoginForm();
     initRecentTab();
     RightSidebarManager.init();
@@ -7829,6 +7832,9 @@
     const authOk = await AuthManager.initAuth();
 
     if (authOk) {
+      // Start SSE sync AFTER auth is established (cookie available)
+      initSyncStatus();
+
       try {
         await Promise.all([loadVaultSettings(), loadVaults(), loadTags()]);