design: Adjust macOS app icon size to follow system standard

[urmauur]

Description:

The current Jan app icon appears visually larger than expected in both the macOS Launchpad and Dock, which breaks the visual consistency with other apps. This likely results from incorrect image padding or misconfigured .icns asset generation.

Issue Details:

• The icon doesn’t match the standard visual scale used by macOS apps.
• This impacts visual polish and platform consistency.

Expected Behavior:

• The app icon should follow macOS Human Interface Guidelines (HIG) for icon sizing.
• The .icns file should include proper padding and scalable layers (16x16 up to 1024x1024) to render cleanly at all system sizes.

Steps to Fix:

• Review and regenerate the icon assets with correct padding and safe area.
• Export .icns tools like iconutil to preserve all sizes.
• Test final appearance in Launchpad and Dock on macOS 12+.

Screenshots: