Custom drawing implementation

lib/features/sheet_viewer/drawing/drawing.dart

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+/// Custom drawing library for PDF annotations.
+///
+/// Provides scalable drawing with normalized coordinates (0-1 range)
+/// that work correctly when the canvas size changes.
+library;
+
+export 'drawing_board.dart';
+export 'drawing_canvas.dart';
+export 'drawing_controller.dart';
+export 'drawing_line.dart';
+export 'drawing_toolbar.dart';
+export 'paint_preset.dart';