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>

packaging/arch/PKGBUILD

@@ -5,7 +5,7 @@
 #   sudo pacman -U predator-control-*.pkg.tar.zst
 
 pkgname=predator-control
-pkgver=1.1.0
+pkgver=1.2.0
 pkgrel=1
 pkgdesc="PredatorSense replacement for Acer Predator laptops: performance modes, fans, keyboard RGB, lightbar, battery (Qt app + Plasma widget)"
 arch=('any')
@@ -22,7 +22,7 @@ install=predator-control.install
 source=()
 
 _src="$(realpath "$startdir/../..")"
-_modver=1.1.0
+_modver=1.2.0
 
 package() {
     cd "$_src"