improved unknown file preview handling with probe failure fallbacks and switched to an explicit TabController in the share view to prevent build-time layout issues

lib/widget/file_viewer.dart

@@ -508,8 +508,6 @@ class _FileViewerState extends State<FileViewer> {
   Widget _buildUnknownPlaceholder() {
     final theme = Theme.of(context);
     final descriptors = _availableActions();
-    final remote = widget.remoteFile;
-    final hasPreview = remote?.hasPreview == true;
     return ListView(
       padding: const EdgeInsets.symmetric(vertical: 24),
       children: [
@@ -517,16 +515,8 @@ class _FileViewerState extends State<FileViewer> {
           padding: const EdgeInsets.symmetric(horizontal: 24),
           child: Column(
             children: [
-              _UnknownPreviewHeader(remoteFile: remote),
+              _UnknownPreviewBlock(remoteFile: widget.remoteFile),
               const SizedBox(height: 16),
-              if (!hasPreview) ...[
-                Text(
-                  'Vorschau nicht verfügbar',
-                  style: theme.textTheme.titleMedium,
-                  textAlign: TextAlign.center,
-                ),
-                const SizedBox(height: 6),
-              ],
               Text(
                 widget.path.split('/').last,
                 style: theme.textTheme.titleSmall?.copyWith(
@@ -607,24 +597,48 @@ class _TextPayload {
   const _TextPayload({required this.content, required this.truncated});
 }
 
-/// Header for the "Vorschau nicht verfügbar" screen: tries to fetch the
-/// Nextcloud thumbnail and shows it mid-sized if available, otherwise
-/// falls back to the generic file icon.
-class _UnknownPreviewHeader extends StatelessWidget {
+/// Header block for the "Vorschau nicht verfügbar" screen.
+///
+/// Two visual modes — kept layout-equivalent so the screen looks identical
+/// whether the server already said "no preview" or the probe failed late:
+///   * **No preview available** (server said no, no remoteFile, or probe
+///     errored): compact "file icon + 'Vorschau nicht verfügbar' text".
+///   * **Preview rendering / loaded**: mid-sized thumbnail without text.
+class _UnknownPreviewBlock extends StatefulWidget {
   final RemoteFileRef? remoteFile;
-  const _UnknownPreviewHeader({required this.remoteFile});
+  const _UnknownPreviewBlock({required this.remoteFile});
 
+  @override
+  State<_UnknownPreviewBlock> createState() => _UnknownPreviewBlockState();
+}
+
+class _UnknownPreviewBlockState extends State<_UnknownPreviewBlock> {
   static const double _previewSize = 180;
+  bool _failed = false;
 
-  Widget _fallbackIcon() =>
-      const Icon(Icons.insert_drive_file_outlined, size: 60);
+  Widget _compact(ThemeData theme) => Column(
+    mainAxisSize: MainAxisSize.min,
+    children: [
+      const Icon(Icons.insert_drive_file_outlined, size: 60),
+      const SizedBox(height: 16),
+      Text(
+        'Vorschau nicht verfügbar',
+        style: theme.textTheme.titleMedium,
+        textAlign: TextAlign.center,
+      ),
+    ],
+  );
 
   @override
   Widget build(BuildContext context) {
-    final remote = remoteFile;
-    // Skip the probe outright when the server already told us there is no
-    // preview — saves an HTTP request that would 404 anyway.
-    if (remote == null || remote.hasPreview == false) return _fallbackIcon();
+    final theme = Theme.of(context);
+    final remote = widget.remoteFile;
+    final canProbe =
+        remote != null &&
+        remote.hasPreview != false &&
+        remote.fileId != null &&
+        !_failed;
+    if (!canProbe) return _compact(theme);
     return SizedBox(
       width: _previewSize,
       height: _previewSize,
@@ -633,10 +647,19 @@ class _UnknownPreviewHeader extends StatelessWidget {
         imageUrl: _ncPreviewUrl(remote, width: 360),
         fadeInDuration: Duration.zero,
         fadeOutDuration: Duration.zero,
-        errorListener: (_) {},
+        // Late probe failure: re-render into the compact layout so the
+        // screen doesn't keep a 180×180 box around a tiny icon. Deferred
+        // to the next frame because setState during build is illegal.
+        errorListener: (_) {
+          if (!mounted) return;
+          WidgetsBinding.instance.addPostFrameCallback((_) {
+            if (mounted) setState(() => _failed = true);
+          });
+        },
         placeholder: (_, _) =>
             const Center(child: AppProgressIndicator.large()),
-        errorWidget: (_, _, _) => Center(child: _fallbackIcon()),
+        // Briefly empty while the post-frame setState swaps layouts.
+        errorWidget: (_, _, _) => const SizedBox.shrink(),
         imageBuilder: (_, imageProvider) => ClipRRect(
           borderRadius: BorderRadius.circular(8),
           child: Image(