General: Fit the app icon to launcher masks with a dark background

[d4rken]

What changed

The launcher icon from #81 is tuned to how launchers actually render it: the mascot no longer touches the circular mask edge (its antennas grazed it), and the icon's background circle is now a muted dark navy instead of bright white, matching how the sibling apps' icons sit on a homescreen. The Play Store icon and the in-app brand mark keep the light tile.

Technical Context

• Verified on a Pixel 9 Pro homescreen next to SD Maid and Butler: at foreground scale 0.70 the antenna tips sat at ~95% of the circle mask's visible radius; 0.64 reproduces the siblings' margins. The monochrome layer moves with it so themed icons match.
• The background layer color (#444E71) is deliberately lighter than the mascot's navy body so the body still reads against it, the same relationship as Butler's black suit on its gray circle.
• Only the three adaptive layers changed; the pre-composed full icon, fastlane icon, and feature graphic are byte-identical (Butler uses the same dark-launcher/light-listing split).
• On-device verification covered the launcher rendering; themed-icon and notification rendering are still preview-verified only.