fix(go): flatten iOS app icon — remove alpha channel for App Store

Apple rejects iOS app icons with an alpha channel ("Invalid large app
icon. The large app icon in the asset catalog ... can't be transparent
or contain an alpha channel"). The 1024×1024 icon was RGBA with the
squircle pre-baked in white + transparent corners + an outer shadow.

Composited over an opaque white background and stripped the alpha
channel. Apple applies the iOS squircle mask itself, so the visible
result is the brand mark on white inside a rounded square — clean.

macOS icons (16–512 variants) are untouched; macOS accepts alpha.