Allow free zoom and pan in drawing mode

lib/features/sheet_viewer/drawing/drawing_board.dart

@@ -96,19 +96,33 @@ class _DrawingBoardState extends State<DrawingBoard> {
     }
 
     // Drawing mode: wrap with InteractiveViewer for zoom/pan
-    return Align(
-      alignment: widget.alignment,
-      child: InteractiveViewer(
+    // Use LayoutBuilder to get available size and center content manually
+    return LayoutBuilder(
+      builder: (context, constraints) {
+        // Calculate padding to center the content
+        final horizontalPadding =
+            (constraints.maxWidth - widget.boardSize.width) / 2;
+        final verticalPadding =
+            (constraints.maxHeight - widget.boardSize.height) / 2;
+
+        return InteractiveViewer(
           transformationController: _transformationController,
           minScale: widget.minScale,
           maxScale: widget.maxScale,
-        boundaryMargin: EdgeInsets.zero,
-        constrained: true,
+          boundaryMargin: const EdgeInsets.all(double.infinity),
+          constrained: false,
           panEnabled: !_isDrawing,
           scaleEnabled: !_isDrawing,
+          child: Padding(
+            padding: EdgeInsets.only(
+              left: horizontalPadding > 0 ? horizontalPadding : 0,
+              top: verticalPadding > 0 ? verticalPadding : 0,
+            ),
             child: content,
           ),
         );
+      },
+    );
   }
 
   /// Builds the drawing layer with gesture handling.