- Add -f/--fail to all curl calls so HTTP error pages (502/404) are not
written to temp files and mistaken for valid config/scripts
- Fix tmux check at line 49: was testing the directory path with -f
(always true when dir exists) causing tmux.conf to re-download on
every shell start; now checks for the actual conf files in both
tmux and byobu config dirs
Adds syntax validation via zsh -n to both update_zshrc() and the
auto-update on shell start. Prevents error pages or corrupted content
from overwriting ~/.zshrc and breaking the shell.