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/Modals/Settings/SettingsSidebar.qml

@@ -164,7 +164,8 @@ Rectangle {
                     "id": "updater",
                     "text": I18n.tr("System Updater"),
                     "icon": "refresh",
-                    "tabIndex": 20
+                    "tabIndex": 20,
+                    "updaterOnly": true
                 },
                 {
                     "id": "desktop_widgets",
@@ -340,6 +341,8 @@ Rectangle {
             return false;
         if (item.clipboardOnly && (!DMSService.isConnected || DMSService.apiVersion < 23))
             return false;
+        if (item.updaterOnly && !SystemUpdateService.sysupdateAvailable)
+            return false;
         return true;
     }