PAINTROID-799 Fix transparency visibility and watercolor zoom darkening

[this-is-varunxyz]

This pull request addresses two distinct UI/UX issues related to transparency and rendering. First, it improves the visibility of transparent colors in the UI by adding a neutral background. Second, it resolves a bug where watercolor strokes would stack opacity and darken whenever the user zoomed or panned the canvas.

Jira Ticket: PAINTROID-799

New Features and Enhancements

(None)

Refactorings and Bug Fixes

Transparency Visibility Fix: Wrapped the color preview squares in the Bottom Navigation Bar and the ColorPicker package with a white background layer to provide a neutral reference for semi-transparent colors.

Watercolor Darkening Fix: Introduced a _hasActiveDrawing flag in the DrawingCanvas to ensure the image cache is only updated after a valid drawing gesture. This prevents "double-painting" the last command during zoom and pan interactions.

Test Infrastructure: Added a regression test in bottom_control_navigation_bar_test.dart and a unit test in watercolor_tool_test.dart to verify constant alpha maintenance.

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