Fix: Use JavaScript form submission for Collabora POST request

- Collabora expects POST to loleaflet.html with WOPISrc in body, not GET
- Previous Dart form.submit() wasn't executing reliably
- Use ScriptElement to inject and execute form submission JavaScript
- This bypasses Dart's HTML layer limitations and ensures form POSTs properly
- Unique viewType names with timestamp to avoid registration conflicts

b0esche_cloud/lib/pages/document_viewer.dart

@@ -459,8 +459,8 @@ class _DocumentViewerModalState extends State<DocumentViewerModal> {
 
   Widget _buildCollaboraIframe(String collaboraUrl) {
     // For Collabora Online, we need to POST the WOPISrc, not GET it
-    // Create an iframe and submit a form to load the document
-    const String viewType = 'collabora-form';
+    // Use JavaScript to create and submit the form properly
+    final String viewType = 'collabora-form-${DateTime.now().millisecondsSinceEpoch}';
 
     ui.platformViewRegistry.registerViewFactory(viewType, (int viewId) {
       // Extract the WOPISrc from the URL
@@ -477,7 +477,7 @@ class _DocumentViewerModalState extends State<DocumentViewerModal> {
 
       // Create the iframe first
       final iframe = html.IFrameElement()
-        ..name = 'collabora-iframe'
+        ..name = 'collabora-iframe-$viewId'
         ..style.border = 'none'
         ..style.width = '100%'
         ..style.height = '100%'
@@ -494,26 +494,36 @@ class _DocumentViewerModalState extends State<DocumentViewerModal> {
 
       container.append(iframe);
 
-      // Create a hidden form that will POST to Collabora
-      final form = html.FormElement()
-        ..method = 'POST'
-        ..action = 'https://of.b0esche.cloud/loleaflet/dist/loleaflet.html'
-        ..target = 'collabora-iframe'
-        ..style.display = 'none';
+      // Use JavaScript to create and submit the form
+      // This is more reliable than Dart's form.submit()
+      final jsCode = '''
+(function() {
+  var form = document.createElement('form');
+  form.method = 'POST';
+  form.action = 'https://of.b0esche.cloud/loleaflet/dist/loleaflet.html';
+  form.target = 'collabora-iframe-$viewId';
+  form.style.display = 'none';
+  
+  var input = document.createElement('input');
+  input.type = 'hidden';
+  input.name = 'WOPISrc';
+  input.value = '$wopisrc';
+  
+  form.appendChild(input);
+  document.body.appendChild(form);
+  
+  // Submit the form
+  setTimeout(function() {
+    form.submit();
+  }, 50);
+})();
+      ''';
 
-      // Add WOPISrc as a hidden input
-      final wopisrcInput = html.InputElement()
-        ..type = 'hidden'
-        ..name = 'WOPISrc'
-        ..value = wopisrc;
-
-      form.append(wopisrcInput);
-      container.append(form);
-
-      // Use a timer to submit the form after the iframe is fully loaded
-      Future.delayed(const Duration(milliseconds: 100), () {
-        form.submit();
-      });
+      final script = html.ScriptElement()
+        ..type = 'text/javascript'
+        ..text = jsCode;
+      
+      html.document.body!.append(script);
 
       return container;
     });