fix: respect sidebar decoration color in macos window (#587)

[rubenRoehner]

Fixes #587.

• Respect Sidebar.decoration.color when provided; otherwise keep the macOS sidebar transparent to preserve blur/tint, and use theme.canvasColor on web.
• Remove the hard-coded black fill that caused a dark strip at the top/bottom when wallpaper tinting was disabled/light mode was active.
• Bump version and update CHANGELOG.

Pre-launch checklist:

• Version bumped and CHANGELOG updated
• Imports organized
• Analyzer clean

[Copilot]

Pull request overview

This PR fixes macOS sidebar background painting in MacosWindow by honoring Sidebar.decoration.color when provided and otherwise preserving native transparency (to keep blur/tint behavior), addressing issue #587. It also bumps the package version and documents the fix in the changelog.

Changes:

• Update MacosWindow sidebar background logic to use Sidebar.decoration.color when set, and keep macOS sidebar transparent by default (with a web fallback to theme.canvasColor).
• Remove the hard-coded black fill that caused visible dark strips when wallpaper tinting was disabled/light mode was active.
• Bump version to 2.2.3 and add a 2.2.3 changelog entry.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
pubspec.yaml	Version bump to 2.2.3.
lib/src/layout/window.dart	Adjust sidebar background painting rules and remove hard-coded black fill.
CHANGELOG.md	Document the sidebar background fix for 2.2.3.

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[Copilot]

In the non-web branch, AnimatedContainer already paints sidebarBackgroundColor via its color:. The new DecoratedBox also applies sidebarBackgroundColor as the BoxDecoration.color, which will effectively paint the color twice when the caller provides a semi-transparent Sidebar.decoration.color (making it appear darker than requested). Consider having only a single widget paint the background color (e.g., move the sidebar.decoration/color onto AnimatedContainer.decoration and drop the inner color paint) so opacity is applied exactly once.

Suggested change

	(sidebar.decoration ?? const BoxDecoration())
	.copyWith(
	// Only paint if the caller set a color; null preserves native transparency.
	color: sidebarBackgroundColor,
	),
	sidebar.decoration ?? const BoxDecoration(),

[Copilot]

This change introduces new sidebar background behavior across platforms (transparent on macOS unless Sidebar.decoration.color is set; theme.canvasColor on web). Since test/layout/window_test.dart already covers MacosWindow/Sidebar, it would be good to add a widget test asserting the expected background paint behavior for (1) no decoration color vs (2) a provided decoration color, to prevent regressions like the black strip issue.