system updater: complete overhaul

Move system update flow to GO, with a CLI (convenient AIO tool) and
server integration. All lifecycle, scheduling, execution occurs on
backend side.

Run some backends via pkexec, some via terminal like paru/yay.

Incorporate flatpak as an option to update.

Add terminal override setting in GUI, in addition to $TERMINAL env
variable.

fixes #2307
fixes #822
fixes #1102
fixes #1812
fixes #1087
fixes #1743

quickshell/Common/SettingsData.qml

@@ -640,6 +640,9 @@ Singleton {
     property bool updaterUseCustomCommand: false
     property string updaterCustomCommand: ""
     property string updaterTerminalAdditionalParams: ""
+    property int updaterIntervalSeconds: 1800
+    property bool updaterIncludeFlatpak: true
+    property bool updaterAllowAUR: true
 
     property string displayNameMode: "system"
     property var screenPreferences: ({})