Per W3C spec, purpose: "monochrome" icons use only the alpha channel
as the silhouette; RGB is ignored and replaced with the platform
theme color. The previous monochrome-icon.png was a black-on-white
RGB image with no alpha, so Safari (macOS PWAs) and Chrome (Android
themed icons) treated every pixel as opaque and painted the whole
1024x1024 canvas with theme_color (#ffa4c0) - a solid pink tile.
Regenerate as RGBA with the silhouette in alpha (derived from the
favicon's alpha channel via a sharp-based helper script). Rename to
monochromeicon.png to bust Safari's stuck PWA icon cache from prior
broken installs.
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