Fix: Use JavaScript in head to submit form for Collabora POST

- Inject script element into head that creates and submits form
- Script executes immediately when loaded, doesn't rely on Future callbacks
- Form targets iframe by name, WOPISrc in body not query params
- This ensures form submission happens reliably before any async operations

b0esche_cloud/lib/pages/document_viewer.dart

@@ -450,14 +450,12 @@ class _DocumentViewerModalState extends State<DocumentViewerModal> {
 
   Widget _buildCollaboraIframe(String wopisrc) {
     // For Collabora Online, POST the WOPISrc to loleaflet.html
-    // We'll create an iframe and submit a form to it from the page level
+    // Use JavaScript to submit the form reliably
     final String viewType =
         'collabora-viewer-${DateTime.now().millisecondsSinceEpoch}';
+    final String iframeName = 'collabora-iframe-$viewType';
 
     ui.platformViewRegistry.registerViewFactory(viewType, (int viewId) {
-      // Create a unique iframe name
-      final iframeName = 'collabora-$viewId';
-
       // Create the iframe that will receive the form submission
       final iframe = html.IFrameElement()
         ..name = iframeName
@@ -484,45 +482,32 @@ class _DocumentViewerModalState extends State<DocumentViewerModal> {
         ..style.overflow = 'hidden'
         ..append(iframe);
 
-      // Create form in the parent document that targets this iframe
+      // Create JavaScript to submit the form
-      // We need to do this after the view is registered but before it's displayed
+      final jsCode = '''
-      // So we use a setTimeout to ensure the DOM is ready
+(function() {
-      Future.delayed(Duration.zero, () {
+  var form = document.createElement('form');
-        // Get or create a hidden form container in the document body
+  form.method = 'POST';
-        final formContainer = html.document.querySelector('#collabora-forms') ??
+  form.action = 'https://of.b0esche.cloud/loleaflet/dist/loleaflet.html';
-            (html.DivElement()
+  form.target = '$iframeName';
-              ..id = 'collabora-forms'
+  form.style.display = 'none';
-              ..style.display = 'none'
-              ..style.position = 'absolute'
-              ..style.width = '0'
-              ..style.height = '0'
-              ..style.overflow = 'hidden');
   
-        if (html.document.querySelector('#collabora-forms') == null) {
+  var input = document.createElement('input');
-          html.document.body!.append(formContainer);
+  input.type = 'hidden';
-        }
+  input.name = 'WOPISrc';
+  input.value = '$wopisrc';
   
-        // Create the form
+  form.appendChild(input);
-        final form = html.FormElement()
+  document.body.appendChild(form);
-          ..id = 'form-$iframeName'
+  form.submit();
-          ..method = 'POST'
+})();
-          ..action = 'https://of.b0esche.cloud/loleaflet/dist/loleaflet.html'
+      ''';
-          ..target = iframeName;
 
-        // Add WOPISrc input
+      // Create and execute script
-        final input = html.InputElement()
+      final script = html.ScriptElement()
-          ..type = 'hidden'
+        ..type = 'text/javascript'
-          ..name = 'WOPISrc'
+        ..text = jsCode;
-          ..value = wopisrc;
 
-        form.append(input);
+      html.document.head!.append(script);
-        formContainer.append(form);
-
-        // Submit the form after a short delay to ensure everything is connected
-        Future.delayed(Duration(milliseconds: 100), () {
-          form.submit();
-        });
-      });
 
       return container;
     });