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/DMSShell.qml

@@ -895,7 +895,12 @@ Item {
 
         SystemUpdatePopout {
             id: systemUpdatePopout
-            onPopoutClosed: PopoutService.unloadSystemUpdate()
+            onPopoutClosed: {
+                if (systemUpdatePopout._reopenAfterUpgrade) {
+                    return;
+                }
+                PopoutService.unloadSystemUpdate();
+            }
 
             Component.onCompleted: {
                 PopoutService.systemUpdatePopout = systemUpdatePopout;