PAINTROID-799 Add white background to color preview for transparency

[this-is-varunxyz]

This pull request addresses the visibility issue of transparent colors in the user interface. Previously, semi-transparent colors in the bottom navigation bar and color picker blended directly with the application's grey/dark theme, leading to inaccurate previews or "muddy" colors.

This is a partial fix for Jira ticket #799.
New Features and Enhancements

Enhanced Color Accuracy: Users can now see a true representation of a color's transparency against a neutral white base.

Refactorings and Bug Fixes

Bottom Navigation Bar: Wrapped the color preview square in a white background container to act as a neutral base for transparency.

Color Picker Package: Updated the ColorSquare component with a Stack containing a white background layer.

Test Infrastructure: Updated BottomNavBarInteractions and added a regression test in bottom_control_navigation_bar_test.dart to verify the white background presence.

Checklist

[x] Include the name of the Jira ticket in the PR’s title

[x] Add the link to the ticket in Jira in the description of the PR (#799)

[x] Include a summary of the changes plus the relevant context

[x] Choose the proper base branch (develop)

[x] Confirm that the changes follow the project’s coding guidelines

[x] Verify that the changes generate no compiler or linter warnings

[x] Perform a self-review of the changes

[x] Verify to commit no other files than the intentionally changed ones

[x] Include reasonable and readable tests verifying the added behavior

[x] Confirm that new and existing tests pass locally

[x] Check that the commits’ message style matches the project’s guideline

[x] Verify that your changes do not have any conflicts with the base branch