v1.19 — Bug fixes, Hungarian accents, better error handling

- Fix: Back button was always visible (ternary always returned Visible)
- Fix: Missing Hungarian accents on EditionPage ("Válasszon kiadást")
- Fix: Better ODT download error messages (network/timeout distinguished)
- Signed with Azure Trusted Signing (Microsoft root CA, instant SmartScreen trust)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

MainWindow.xaml.cs

@@ -121,7 +121,7 @@ namespace InstaSoftOfficeTool
 
         private void UpdateButtons()
         {
-            BtnBack.Visibility = _currentPageIndex > 0 ? Visibility.Visible : Visibility.Visible;
+            BtnBack.Visibility = _currentPageIndex > 0 ? Visibility.Visible : Visibility.Collapsed;
             BtnNext.Visibility = Visibility.Visible;
 
             // On SummaryPage = "Tov\u00e1bb"