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/Modules/Settings/LauncherTab.qml

@@ -325,6 +325,8 @@ Item {
                     placeholderText: I18n.tr("Enter launch prefix (e.g., 'uwsm-app')")
                     onTextEdited: SettingsData.set("launchPrefix", text)
                 }
+
+                TerminalPickerRow {}
             }
 
             SettingsCard {