Custom drawing implementation

lib/features/sheet_viewer/drawing/paint_preset.dart

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+import 'package:flutter/material.dart';
+
+/// Predefined paint configurations for common annotation styles.
+///
+/// Each preset defines a color and stroke width for drawing.
+/// Stroke width is normalized (relative to canvas width).
+class PaintPreset {
+  /// Display name for the preset
+  final String name;
+
+  /// Color of the paint (including opacity)
+  final Color color;
+
+  /// Stroke width in normalized units (relative to canvas width)
+  /// A value of 0.005 means the stroke is 0.5% of the canvas width
+  final double strokeWidth;
+
+  /// Icon to display for this preset
+  final IconData icon;
+
+  const PaintPreset({
+    required this.name,
+    required this.color,
+    required this.strokeWidth,
+    required this.icon,
+  });
+
+  // ---------------------------------------------------------------------------
+  // Default Presets
+  // ---------------------------------------------------------------------------
+
+  /// Black pen for writing/notes
+  static const blackPen = PaintPreset(
+    name: 'Black Pen',
+    color: Colors.black,
+    strokeWidth: 0.003, // Thin line for writing
+    icon: Icons.edit,
+  );
+
+  /// Red pen for corrections/markings
+  static const redPen = PaintPreset(
+    name: 'Red Pen',
+    color: Colors.red,
+    strokeWidth: 0.003,
+    icon: Icons.edit,
+  );
+
+  /// Blue pen for annotations
+  static const bluePen = PaintPreset(
+    name: 'Blue Pen',
+    color: Colors.blue,
+    strokeWidth: 0.003,
+    icon: Icons.edit,
+  );
+
+  /// Yellow highlighter (semi-transparent, thicker)
+  static const yellowMarker = PaintPreset(
+    name: 'Yellow Marker',
+    color: Color(0x80FFEB3B), // Yellow with 50% opacity
+    strokeWidth: 0.015, // Thicker for highlighting
+    icon: Icons.highlight,
+  );
+
+  /// Green highlighter
+  static const greenMarker = PaintPreset(
+    name: 'Green Marker',
+    color: Color(0x804CAF50), // Green with 50% opacity
+    strokeWidth: 0.015,
+    icon: Icons.highlight,
+  );
+
+  /// Pink highlighter
+  static const pinkMarker = PaintPreset(
+    name: 'Pink Marker',
+    color: Color(0x80E91E63), // Pink with 50% opacity
+    strokeWidth: 0.015,
+    icon: Icons.highlight,
+  );
+
+  /// All available default presets
+  static const List<PaintPreset> defaults = [
+    blackPen,
+    redPen,
+    bluePen,
+    yellowMarker,
+    greenMarker,
+    pinkMarker,
+  ];
+
+  /// Quick access presets (shown in main toolbar)
+  static const List<PaintPreset> quickAccess = [
+    blackPen,
+    redPen,
+    yellowMarker,
+  ];
+
+  @override
+  bool operator ==(Object other) {
+    if (identical(this, other)) return true;
+    return other is PaintPreset &&
+        other.name == name &&
+        other.color == color &&
+        other.strokeWidth == strokeWidth;
+  }
+
+  @override
+  int get hashCode => Object.hash(name, color, strokeWidth);
+}