chore: store icon.png in LFS as .gitattributes declares

icon.png was committed as a raw blob while .gitattributes marks *.png as
filter=lfs. The clean filter therefore produced a pointer that never matched
the stored blob, so git reported the file as permanently modified whenever
the stat cache was invalidated, which blocked rebases. Renormalise it to an
actual LFS pointer. File content is unchanged - a fresh clone smudges back
to a byte-identical PNG.

Author: matt-edmondson