Fix Collabora Online loading using form-based POST instead of URL params

- Collabora's loleaflet.html expects WOPISrc as POST parameter, not URL query param
- Changed from iframe with src= to form submission approach
- Extract WOPISrc from URL and pass as hidden form input
- This avoids 400 Bad Request from Collabora when using GET with URL params

b0esche_cloud/lib/pages/document_viewer.dart

@@ -404,8 +404,9 @@ class _DocumentViewerModalState extends State<DocumentViewerModal> {
         final wopiSession = snapshot.data!;
 
         // Build Collabora Online viewer URL with WOPISrc
+        // Note: Don't double-encode the WOPISrc - Collabora expects the URL as-is
         final collaboraUrl =
-            'https://of.b0esche.cloud/loleaflet/dist/loleaflet.html?WOPISrc=${Uri.encodeComponent(wopiSession.wopisrc)}';
+            'https://of.b0esche.cloud/loleaflet/dist/loleaflet.html?WOPISrc=${wopiSession.wopisrc}';
 
         // Use WebView to display Collabora Online
         return _buildWebView(collaboraUrl);
@@ -453,31 +454,69 @@ class _DocumentViewerModalState extends State<DocumentViewerModal> {
   }
 
   Widget _buildCollaboraIframe(String collaboraUrl) {
-    // Register the iframe element
+    // For Collabora Online, we need to use a form-based approach instead of iframe
-    const String viewType = 'collabora-iframe';
+    // because loleaflet.html expects a POST request with WOPISrc in the body
     
-    // Create the iframe with proper attributes for Collabora Online
+    const String viewType = 'collabora-form-container';
-    ui.platformViewRegistry.registerViewFactory(viewType, (int viewId) {
+    
-      final iframe = html.IFrameElement()
+    ui.platformViewRegistry.registerViewFactory(
-        ..src = collaboraUrl
+      viewType,
-        ..style.border = 'none'
+      (int viewId) {
-        ..style.width = '100%'
+        // Create a container div for the form and iframe
-        ..style.height = '100%'
+        final container = html.DivElement()
-        ..style.margin = '0'
+          ..style.width = '100%'
-        ..style.padding = '0'
+          ..style.height = '100%'
-        ..setAttribute(
+          ..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] 
+            : '';
+        
+        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';
+        
+        // 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',
+        );
+        
+        iframe.setAttribute(
           'allow',
           'microphone; camera; usb; autoplay; clipboard-read; clipboard-write',
         );
         
-      // Prevent sandbox restrictions that might block Collabora
+        container.append(iframe);
-      iframe.setAttribute(
-        'sandbox',
-        'allow-same-origin allow-scripts allow-popups allow-forms allow-top-navigation allow-pointer-lock allow-presentation allow-modals',
-      );
         
-      return iframe;
+        // Submit the form to load the document
-    });
+        form.submit();
+        
+        return container;
+      },
+    );
 
     return HtmlElementView(viewType: viewType);
   }