Add built-in updater, bump to 1.2.0

- predatord checks the Gitea releases every 6 hours, downloads the
  newest pacman package, verifies its SHA-256 from the release notes
  and installs it with `pacman -U` in a separate systemd unit
- Update notice and install button on the System page, in the sidebar
  and in the panel widget; `predatorctl update [--check] [--yes]`
- Optional API token for private repositories
- packaging/release.py builds and publishes releases in the format
  the updater expects

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

install.sh

@@ -4,7 +4,7 @@
 #   sudo ./install.sh --no-deps  skip package installation (non-Arch distros)
 set -euo pipefail
 
-VERSION="1.1.0"
+VERSION="1.2.0"
 MODULE="predator-wmi"
 SRC="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 PREFIX="/usr"