assets/logo.png had NO real alpha transparency — both the background and
the eagle shape were fully opaque (A=255 everywhere), just baked in as
near-black (3,3,3) vs near-white (253,253,253) RGB. Applying `color:
Colors.white` to tint it painted the WHOLE bounding square white (alpha
being 255 across the entire image gives BlendMode nothing to mask against),
which at low Opacity looked like a flat gray square instead of a silhouette.
Converted the asset in place: since it was already grayscale (R=G=B), each
pixel's luminance became its new alpha channel, RGB set to pure white. The
background (near-black, low luminance) is now near-transparent; the eagle
(near-white, high luminance) is now near-opaque. This is the same effect the
desktop GUI gets for free from its logo.svg (a vector eagle path with no
background element at all — inherently transparent), just reproduced for a
raster asset without adding flutter_svg as a new dependency.
The `color: Colors.white` tint in both watermark call sites is now
redundant (the asset is already a pure-white silhouette) and removed.
ospab