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/Widgets/TerminalPickerRow.qml

@@ -0,0 +1,31 @@
+import QtQuick
+import qs.Common
+
+SettingsDropdownRow {
+    id: root
+
+    readonly property string autoLabel: I18n.tr("Auto")
+
+    text: I18n.tr("Terminal")
+    settingKey: "terminalOverride"
+
+    options: {
+        const opts = [autoLabel];
+        const installed = SessionData.installedTerminals || [];
+        const list = installed.length > 0 ? installed : SessionData.terminalOptions;
+        for (const t of list) {
+            opts.push(t);
+        }
+        if (SessionData.terminalOverride && !opts.includes(SessionData.terminalOverride)) {
+            opts.push(SessionData.terminalOverride);
+        }
+        return opts;
+    }
+
+    currentValue: SessionData.terminalOverride.length > 0 ? SessionData.terminalOverride : autoLabel
+
+    onValueChanged: label => {
+        const next = label === autoLabel ? "" : label;
+        SessionData.set("terminalOverride", next);
+    }
+}