Harden install and runtime for production use

[pidoshva]

Makes the install one-liner and the runtime robust enough for anyone to install and run, with dependencies installed and requirements enforced.

Fixes

Critical — spinner crashed under zsh
The frame-cycling substring ${frames:i%${#frames}:1} made zsh throw unrecognized modifier 'i' on every run. zsh is the default shell on modern macOS — exactly where install.sh sources the script — so every Mac user saw the error (the GIF still produced, but with a visible error spewed each run). Reworked to use arithmetic expansion, which parses correctly in both bash and zsh.

--fps input validation
--fps abc / --fps 0 previously passed straight to ffmpeg and failed mid-encode with a cryptic message. Now validated as a positive integer up front.

Installer hardening (install.sh)

• Verify ffmpeg is actually on PATH after the install step (a package manager can exit 0 without exposing the binary).
• Enforce ffprobe (used for input analysis; normally bundled with ffmpeg).
• Check for curl before the remote download path, with a clear fallback message.
• Verify the installed gifify.sh is non-empty before activating it; make the optional VERSION download non-fatal.

Verification

• shellcheck clean on both scripts.
• Sourced and ran a real conversion under both bash and zsh — zsh is now error-free.
• Full install → run → uninstall cycle in an isolated HOME under zsh.