PAINTROID-737 Add pipette tool to the colorpicker package

[Amit-Matth]

This PR adds a pipette tool to the color picker, allowing users to pick a color directly from the canvas.

PAINTROID-737

New Features and Enhancements

As present in the native application, a pipette tool for selecting a particular color from the current state of the drawing canvas is implemented. This tool is a part of the colorpicker package itself and allows the user to pick any color from the drawing and use it on other objects.

• Pipette Integration: Added PipetteToolButton to the ColorPicker UI, enabling navigation to the color selection interface.
• Pipette Page: Developed PipettePage to render a screen snapshot and extract colors using ui.Image byte data.
• Precision Loupe: Implemented a magnifying loupe with a crosshair for accurate color picking during pan gestures.
• Confirmation Flow: Added a "Save changes?" dialog to ensure picked colors are not lost when navigating back without confirmation.
• Testing: Created a robust test suite in pipette_button_test.dart covering UI alignment, navigation, color selection, loupe visibility, and dialog interactions.

Refactorings and Bug Fixes

• Refactorings and Bug Fixes

Checklist

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