fix: dnsmasq path detection and WiFi rfkill persistence

- dns-hijack.js: replace `which` with multi-path search (/usr/sbin etc.)
  since dnsmasq is often not in normal user PATH
- install.sh: auto-enable NetworkManager for WiFi provisioning
- install.sh: unblock WiFi rfkill at install time
- install.sh: create clawd-rfkill.service for persistent boot-time unlock