bug(cli): upgrade crashes when backup target doesn't exist in --install-dir

[rachmlenig]

Description

lf upgrade --install-dir ~/.local/bin crashes when there's no existing lf binary in the target directory. The upgrade process tries to back up a file that doesn't exist before replacing it.

Expected Behavior

If no existing binary is found in the install directory, skip the backup step and proceed with the install.

Actual Behavior

The upgrade crashes trying to back up a non-existent file.

Workaround

Manually copy the current binary to the target directory before running upgrade:

cp $(which lf) ~/.local/bin/lf
lf upgrade --install-dir ~/.local/bin

Environment

• OS: Ubuntu (Linux)