Fix Collabora 400 error by properly URL-encoding WOPISrc parameter

- WOPISrc is a full URL that needs URL encoding when passed as query param
- Use Uri.encodeComponent() to properly encode the WOPISrc value
- Simplify iframe setup to just use the properly constructed URL
- This fixes the 400 Bad Request from Collabora when loading documents

b0esche_cloud/lib/pages/document_viewer.dart

@@ -404,9 +404,10 @@ 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
+        // The WOPISrc must be URL-encoded since it contains special characters (://, ?, =, &)
+        final encodedWopisrc = Uri.encodeComponent(wopiSession.wopisrc);
         final collaboraUrl =
-            'https://of.b0esche.cloud/loleaflet/dist/loleaflet.html?WOPISrc=${wopiSession.wopisrc}';
+            'https://of.b0esche.cloud/loleaflet/dist/loleaflet.html?WOPISrc=$encodedWopisrc';
 
         // Use WebView to display Collabora Online
         return _buildWebView(collaboraUrl);
@@ -460,13 +461,9 @@ class _DocumentViewerModalState extends State<DocumentViewerModal> {
     ui.platformViewRegistry.registerViewFactory(
       viewType,
       (int viewId) {
-        // Extract the WOPISrc value
-        final wopisrcValue = collaboraUrl.contains('WOPISrc=')
-            ? Uri.decodeComponent(collaboraUrl.split('WOPISrc=')[1])
-            : '';
-        
-        // Create the iframe
+        // Create the iframe with the properly encoded Collabora URL
         final iframe = html.IFrameElement()
+          ..src = collaboraUrl
           ..style.border = 'none'
           ..style.width = '100%'
           ..style.height = '100%'
@@ -476,17 +473,12 @@ class _DocumentViewerModalState extends State<DocumentViewerModal> {
             'allow',
             'microphone; camera; usb; autoplay; clipboard-read; clipboard-write',
           )
-          // Remove allow-same-origin and add allow-popups-to-escape-sandbox for better security
+          // Remove allow-same-origin for security, add allow-popups-to-escape-sandbox
           ..setAttribute(
             'sandbox',
             'allow-scripts allow-popups allow-forms allow-pointer-lock allow-presentation allow-modals allow-downloads allow-popups-to-escape-sandbox',
           );
         
-        // Build the full Collabora URL
-        final collaboraPath =
-            'https://of.b0esche.cloud/loleaflet/dist/loleaflet.html';
-        iframe.src = '$collaboraPath?WOPISrc=$wopisrcValue';
-        
         return iframe;
       },
     );