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>

plasmoid/org.predatorcontrol.panel/metadata.json

@@ -12,7 +12,7 @@
         "Id": "org.predatorcontrol.panel",
         "License": "GPL-2.0-only",
         "Name": "Predator Control",
-        "Version": "1.1.0",
+        "Version": "1.2.0",
         "Website": "https://git.mrblake.cc/MrBlake/Cachy-Predator-Keyboard"
     },
     "X-Plasma-API-Minimum-Version": "6.0",