feat(assets): white monochrome launcher icon

The app icon is a dark glyph on transparency — invisible on dark launcher
backgrounds. icon-white.png keeps the exact silhouette/alpha with opaque
pixels recolored white; generated by make_icon_white.py. Used by the
desktop entry (launcher/taskbar); in-app artwork unchanged.

frontend/public/make_icon_white.py

@@ -0,0 +1,9 @@
+"""Generate icon-white.png: same silhouette as icon.png, opaque pixels
+recolored to white, alpha preserved. Run from frontend/public."""
+from PIL import Image
+
+im = Image.open("icon.png").convert("RGBA")
+out = Image.new("RGBA", im.size)
+out.putdata([(255, 255, 255, a) for _, _, _, a in im.getdata()])
+out.save("icon-white.png")
+print("wrote icon-white.png", out.size)