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/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: {} },