PAINTROID-791 Add pipette tool

[Amit-Matth]

Added pipette tool with magnifier support, real-time color picking, and full undo/redo integration with tests.

PAINTROID-791

New Features and Enhancements

🎨 Pipette Tool

• Pick colors directly from the canvas by tapping any pixel
• No page switching, seamless, in-canvas interaction (Paintroid-like UX)
• Selected color is applied instantly to the paint provider

🔍 Magnifier Glass

• Zoomed-in view under finger/cursor for precise picking
• Follows interaction point and positions above the finger
• Uses ValueNotifier for smooth, efficient updates
• Auto-hides on finger release

↩️ Undo / Redo Support

• Added ColorChangedCommand to track color changes
• Integrated with command manager for proper undo/redo
• Preserves initial color state for accurate rollbacks

Refactorings and Bug Fixes

🔧 Command System

• Updated command factory to support ColorChangedCommand
• Proper command registration for state management
• Improved paint provider integration with pipette tool

🧪 Testing

• Unit tests for pipette tool (pipette_tool_test.dart)
• Widget tests for magnifier glass (magnifier_glass_test.dart)
• Added FakeToolBoxStateProvider for isolated tests
• Verified undo/redo behavior coverage

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