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>

predator_control/__init__.py

@@ -2,7 +2,7 @@
 
 import os
 
-__version__ = "1.1.0"
+__version__ = "1.2.0"
 
 APP_NAME = "Predator Control"
 RUN_DIR = os.environ.get("PREDATOR_CONTROL_RUN_DIR", "/run/predator-control")