[client] Debug: create GUI windows on demand and destroy them on close

Experiment on top of the memory profiler: the main and Settings windows are no
longer created at startup or kept alive hidden, so an idle tray holds no webview
process. The WindowManager owns creation, rebuilds the main window on the next
show, and defers every show until the frontend reports it has rendered, which
keeps an empty webview off screen.

Measured on macOS: idle footprint drops from ~160 MB to ~74 MB, at the cost of
~150-250 ms per window open.

client/ui/tray_update.go

@@ -19,7 +19,7 @@ import (
 // trayUpdater owns the tray UI that reacts to auto-update. Composed inside Tray.
 type trayUpdater struct {
 	app          *application.App
-	window       *application.WebviewWindow
+	showMainAt   func(url string)
 	update       *services.Update
 	notifier     *Notifier
 	loc          *Localizer
@@ -36,10 +36,10 @@ type trayUpdater struct {
 	progressWindowOpen bool
 }
 
-func newTrayUpdater(app *application.App, window *application.WebviewWindow, update *services.Update, notifier *Notifier, loc *Localizer, onIconChange func(), onMenuChange func()) *trayUpdater {
+func newTrayUpdater(app *application.App, showMainAt func(url string), update *services.Update, notifier *Notifier, loc *Localizer, onIconChange func(), onMenuChange func()) *trayUpdater {
 	u := &trayUpdater{
 		app:          app,
-		window:       window,
+		showMainAt:   showMainAt,
 		update:       update,
 		notifier:     notifier,
 		loc:          loc,
@@ -185,14 +185,12 @@ func (u *trayUpdater) sendUpdateNotification(st updater.State) {
 // openProgressWindow points the main window at the /update progress page and
 // brings it forward.
 func (u *trayUpdater) openProgressWindow(version string) {
-	if u.window == nil {
+	if u.showMainAt == nil {
 		return
 	}
 	url := "/#/update"
 	if version != "" {
 		url += "?version=" + version
 	}
-	u.window.SetURL(url)
-	u.window.Show()
-	u.window.Focus()
+	u.showMainAt(url)
 }