Improve error messages for impossible actions during paint mode

lib/features/sheet_viewer/sheet_viewer_page.dart

@@ -92,6 +92,10 @@ class _SheetViewerPageState extends State<SheetViewerPage>
   }
 
   void _toggleFullscreen() {
+    if (_isPaintMode) {
+      _showSnackBar('Cannot enter fullscreen while in paint mode');
+      return;
+    }
     FullScreen.setFullScreen(!widget.config.fullscreen);
   }
 
@@ -111,13 +115,7 @@ class _SheetViewerPageState extends State<SheetViewerPage>
 
   void _togglePaintMode() {
     if (widget.config.twoPageMode) {
-      // Paint mode only works in single page mode
-      ScaffoldMessenger.of(context).showSnackBar(
-        const SnackBar(
-          content: Text('Paint mode is only available in single page mode'),
-          duration: Duration(seconds: 2),
-        ),
-      );
+      _showSnackBar('Paint mode is only available in single page mode');
       return;
     }
 
@@ -125,19 +123,14 @@ class _SheetViewerPageState extends State<SheetViewerPage>
   }
 
   void _toggleTwoPageMode() {
+    if (_isPaintMode) {
+      _showSnackBar('Cannot enter two-page mode while painting');
+      return;
+    }
+
     setState(() {
       widget.config.twoPageMode = !widget.config.twoPageMode;
       _storageService.writeConfig(widget.config);
-
-      if (widget.config.twoPageMode && _isPaintMode) {
-        _isPaintMode = false;
-        ScaffoldMessenger.of(context).showSnackBar(
-          const SnackBar(
-            content: Text('Paint mode disabled in two-page mode'),
-            duration: Duration(seconds: 2),
-          ),
-        );
-      }
     });
   }
 
@@ -252,4 +245,10 @@ class _SheetViewerPageState extends State<SheetViewerPage>
       ),
     );
   }
+
+  void _showSnackBar(String message) {
+    ScaffoldMessenger.of(context).showSnackBar(
+      SnackBar(content: Text(message), duration: Duration(seconds: 2)),
+    );
+  }
 }