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:

• [x] Version bumped and CHANGELOG updated
• [x] Imports organized
• [x] Analyzer clean