Complete refactor to clean up project

lib/features/sheet_viewer/widgets/paint_mode_layer.dart

@@ -0,0 +1,42 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_drawing_board/flutter_drawing_board.dart';
+
+import 'pdf_page_display.dart';
+
+/// Drawing overlay for annotating PDF pages.
+///
+/// Uses flutter_drawing_board to provide a paint canvas over the PDF.
+/// Only available in single-page mode.
+class PaintModeLayer extends StatelessWidget {
+  final PdfPageDisplay pageDisplay;
+
+  const PaintModeLayer({
+    super.key,
+    required this.pageDisplay,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return SizedBox.expand(
+      child: LayoutBuilder(
+        builder: (context, constraints) {
+          final maxSize = Size(constraints.maxWidth, constraints.maxHeight);
+          final (pageSize, _) = pageDisplay.calculateScaledPageSizes(maxSize);
+
+          return DrawingBoard(
+            background: SizedBox(
+              width: pageSize.width,
+              height: pageSize.height,
+              child: pageDisplay,
+            ),
+            boardConstrained: true,
+            minScale: 1,
+            maxScale: 3,
+            alignment: Alignment.topRight,
+            boardBoundaryMargin: EdgeInsets.zero,
+          );
+        },
+      ),
+    );
+  }
+}