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/SessionData.qml

@@ -30,9 +30,36 @@ Singleton {
     property bool isLightMode: false
     property bool doNotDisturb: false
     property real doNotDisturbUntil: 0
+    property string terminalOverride: ""
     property bool isSwitchingMode: false
     property bool suppressOSD: true
 
+    readonly property var terminalOptions: ["ghostty", "kitty", "foot", "alacritty", "wezterm", "konsole", "gnome-terminal", "xterm"]
+    property var installedTerminals: []
+
+    function resolveTerminal() {
+        if (terminalOverride && terminalOverride.length > 0) {
+            return terminalOverride;
+        }
+        const env = Quickshell.env("TERMINAL");
+        if (env && env.length > 0) {
+            return env;
+        }
+        return "";
+    }
+
+    Process {
+        id: terminalProbe
+        running: true
+        command: ["sh", "-c", "for t in ghostty kitty foot alacritty wezterm konsole gnome-terminal xterm; do command -v \"$t\" >/dev/null 2>&1 && echo \"$t\"; done"]
+        stdout: StdioCollector {
+            onStreamFinished: {
+                const found = text.trim().split("\n").filter(line => line.length > 0);
+                root.installedTerminals = found;
+            }
+        }
+    }
+
     Timer {
         id: dndExpireTimer
         repeat: false

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: ({})

quickshell/Common/settings/SessionSpec.js

@@ -4,6 +4,7 @@ var SPEC = {
     isLightMode: { def: false },
     doNotDisturb: { def: false },
     doNotDisturbUntil: { def: 0 },
+    terminalOverride: { def: "" },
 
     wallpaperPath: { def: "" },
     perMonitorWallpaper: { def: false },

quickshell/Common/settings/SettingsSpec.js

@@ -428,6 +428,9 @@ var SPEC = {
     updaterUseCustomCommand: { def: false },
     updaterCustomCommand: { def: "" },
     updaterTerminalAdditionalParams: { def: "" },
+    updaterIntervalSeconds: { def: 1800 },
+    updaterIncludeFlatpak: { def: true },
+    updaterAllowAUR: { def: true },
 
     displayNameMode: { def: "system" },
     screenPreferences: { def: {} },