Fix Collabora iframe loading and sandbox security

- Reverted to direct iframe src approach instead of form submission
- This avoids 'Form submission canceled' error
- Removed allow-same-origin from sandbox to improve security
- Added allow-popups-to-escape-sandbox for Collabora functionality
- Use direct URL with WOPISrc parameter instead of POST form

b0esche_cloud/lib/pages/document_viewer.dart

@@ -454,67 +454,40 @@ class _DocumentViewerModalState extends State<DocumentViewerModal> {
   }
 
   Widget _buildCollaboraIframe(String collaboraUrl) {
-    // For Collabora Online, we need to use a form-based approach instead of iframe
-    // because loleaflet.html expects a POST request with WOPISrc in the body
-    
-    const String viewType = 'collabora-form-container';
+    // For Collabora Online, create an iframe that loads the editor directly
+    const String viewType = 'collabora-iframe';
     
     ui.platformViewRegistry.registerViewFactory(
       viewType,
       (int viewId) {
-        // Create a container div for the form and iframe
-        final container = html.DivElement()
-          ..style.width = '100%'
-          ..style.height = '100%'
-          ..style.margin = '0'
-          ..style.padding = '0';
-        
-        // Create a hidden form to POST to Collabora
-        final form = html.FormElement()
-          ..method = 'POST'
-          ..action = 'https://of.b0esche.cloud/loleaflet/dist/loleaflet.html'
-          ..target = 'collabora-frame'
-          ..style.display = 'none';
-        
-        // Extract the WOPISrc value from the URL
-        final wopisrcValue = collaboraUrl.split('WOPISrc=').length > 1 
-            ? collaboraUrl.split('WOPISrc=')[1] 
+        // Extract the WOPISrc value
+        final wopisrcValue = collaboraUrl.contains('WOPISrc=')
+            ? Uri.decodeComponent(collaboraUrl.split('WOPISrc=')[1])
             : '';
         
-        final wopisrcInput = html.InputElement()
-          ..type = 'hidden'
-          ..name = 'WOPISrc'
-          ..value = wopisrcValue;
-        
-        form.append(wopisrcInput);
-        container.append(form);
-        
         // Create the iframe
         final iframe = html.IFrameElement()
-          ..name = 'collabora-frame'
           ..style.border = 'none'
           ..style.width = '100%'
           ..style.height = '100%'
           ..style.margin = '0'
-          ..style.padding = '0';
+          ..style.padding = '0'
+          ..setAttribute(
+            'allow',
+            'microphone; camera; usb; autoplay; clipboard-read; clipboard-write',
+          )
+          // Remove allow-same-origin and add allow-popups-to-escape-sandbox for better security
+          ..setAttribute(
+            'sandbox',
+            'allow-scripts allow-popups allow-forms allow-pointer-lock allow-presentation allow-modals allow-downloads allow-popups-to-escape-sandbox',
+          );
         
-        // Set sandbox attributes
-        iframe.setAttribute(
-          'sandbox',
-          'allow-same-origin allow-scripts allow-popups allow-forms allow-top-navigation allow-pointer-lock allow-presentation allow-modals allow-downloads',
-        );
+        // Build the full Collabora URL
+        final collaboraPath =
+            'https://of.b0esche.cloud/loleaflet/dist/loleaflet.html';
+        iframe.src = '$collaboraPath?WOPISrc=$wopisrcValue';
         
-        iframe.setAttribute(
-          'allow',
-          'microphone; camera; usb; autoplay; clipboard-read; clipboard-write',
-        );
-        
-        container.append(iframe);
-        
-        // Submit the form to load the document
-        form.submit();
-        
-        return container;
+        return iframe;
       },
     );