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>

README.md

@@ -48,7 +48,7 @@ Download `predator-control-<version>-any.pkg.tar.zst` from the
 
 ```bash
 sudo pacman -S --needed linux-cachyos-headers clang lld llvm   # headers for your kernel (stock Arch: linux-headers)
-sudo pacman -U predator-control-1.1.0-1-any.pkg.tar.zst
+sudo pacman -U predator-control-*-any.pkg.tar.zst
 ```
 
 The service starts automatically and the kernel module is built by DKMS for every installed kernel.
@@ -83,6 +83,16 @@ Keyboard RGB and performance profiles work without the module.
 
 Uninstall with `sudo pacman -R predator-control` (package) or `sudo ./uninstall.sh` (add `--purge` to also delete your settings).
 
+## Updates
+
+Installed via the pacman package, Predator Control updates itself: the service checks for new
+releases every 6 hours, and a notice with an *Install* button appears on the **System** page, in the
+sidebar and in the panel widget. Downloads are verified with the SHA-256 from the release notes and
+installed with `pacman -U`. From the terminal: `predatorctl update` (or `--check`).
+Automatic checks can be turned off on the System page.
+
+If you installed with `install.sh`, update with `git pull && sudo ./install.sh`.
+
 ## Usage
 
 ### Panel widget
@@ -108,6 +118,7 @@ predatorctl kbd-brightness +25
 predatorctl lightbar wave --color 00d4ff --speed 7
 predatorctl lightbar --sync on            # lightbar follows the keyboard
 predatorctl battery-limit on
+predatorctl update                        # check for and install updates
 predatorctl open rgb                      # open the app on a page
 ```