fix(rancher-desktop): use pre-resized 512x512 icon source file

GdkPixbuf runtime resize fails in the flatpak build sandbox
('Loader process exited early with status 1'). Instead, commit a
pre-resized 512x512 PNG and install it directly.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: castrojo