feat: add macOS support

[prdai]

macOS support for zen-sync: profile path under ~/Library/Application Support/zen/Profiles, zen process name (zen vs zen-bin), open -a Zen for launching, liblz4.dylib fallbacks for merge/status, and a python3 fallback for numfmt (not available on stock macOS).

Trimmed per maintainer feedback to macOS-only changes — the age passphrase fix, pull guard, and flatpak path were removed and can go in separate PRs.

[enisbu]

Hey @prdai, thanks for this, and for marking it a draft.

The macOS parts are what the repo needs: profile path, process name, open -a Zen, and the liblz4/numfmt fallbacks. Those I'd take.

The age passphrase rewrite is a different story. You're right that AGE_PASSPHRASE isn't something age reads, so that mode was likely broken from day one, but it touches R2 on every platform and deserves its own PR. Same for the pull guard and the flatpak path.

Would you be up for trimming this one down to the macOS bits? @GerritWanderer runs macOS and Omarchy and offered to test.

[prdai]

hi, i re scoped the pr to only for the macos support required, let me know if there is anything else to be done, tnx

[GerritWanderer]

Hi folks and thanks @prdai for the draft PR. I'll test your branch locally with my Macbook and PC (running Omarchy)