optimized avatar and linkify performance, refined navigation to preserve popups, implemented read marker caching, and added file size limits for saving, minor timetable details changes

lib/widget/file_viewer.dart

@@ -254,7 +254,26 @@ class _FileViewerState extends State<FileViewer> {
         break;
       case FileViewingActions.save:
         try {
-          final bytes = await File(widget.path).readAsBytes();
+          final source = File(widget.path);
+          final size = await source.length();
+          // Hard-cap to avoid loading the entire file into memory just to
+          // hand it back to the platform's saveFile dialog. The package
+          // currently has no streaming/path-based save path, so for big
+          // media the user has to fall back to "Teilen" → save-to-files.
+          // 200 MB peak is comfortable on modern mid-range devices and big
+          // enough for typical school videos.
+          const maxBytes = 200 * 1024 * 1024; // 200 MB
+          if (size > maxBytes) {
+            if (!mounted) return;
+            InfoDialog.show(
+              context,
+              'Diese Datei ist zu groß (${(size / (1024 * 1024)).toStringAsFixed(0)} MB), '
+              'um direkt gespeichert zu werden. Nutze stattdessen die Teilen-Funktion.',
+              title: 'Speichern nicht möglich',
+            );
+            return;
+          }
+          final bytes = await source.readAsBytes();
           final saved = await FilePicker.saveFile(
             fileName: widget.path.split('/').last,
             bytes: bytes,