[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.go

@@ -174,7 +174,7 @@ func NewTray(app *application.App, window *application.WebviewWindow, svc TraySe
 		// in the right locale — no English flash then re-paint.
 		loc: svc.Localizer,
 	}
-	t.updater = newTrayUpdater(app, window, svc.Update, svc.Notifier, t.loc, func() { t.applyIcon() }, func() { t.relayoutMenu() })
+	t.updater = newTrayUpdater(app, t.showMainAt, svc.Update, svc.Notifier, t.loc, func() { t.applyIcon() }, func() { t.relayoutMenu() })
 	t.tray = app.SystemTray.New()
 	// Seed panel-theme detection before the first paint so the initial icon
 	// matches the panel's light/dark scheme (Linux only).
@@ -241,9 +241,6 @@ func (t *Tray) ShowWindow() {
 		w.Focus()
 		return
 	}
-	if t.window == nil {
-		return
-	}
 	// Route through WindowManager so the main window is centered on first
 	// show — minimal WMs (fluxbox, the XEmbed tray path) otherwise drop it in
 	// the top-left corner.
@@ -251,8 +248,40 @@ func (t *Tray) ShowWindow() {
 		t.svc.WindowManager.ShowMain()
 		return
 	}
-	t.window.Show()
-	t.window.Focus()
+	if w := t.mainWindow(); w != nil {
+		w.Show()
+		w.Focus()
+	}
+}
+
+func (t *Tray) mainWindow() *application.WebviewWindow {
+	if t.svc.WindowManager == nil {
+		return t.window
+	}
+	return t.svc.WindowManager.MainWindow()
+}
+
+func (t *Tray) showMain() {
+	if t.svc.WindowManager != nil {
+		t.svc.WindowManager.ShowMain()
+		return
+	}
+	if w := t.mainWindow(); w != nil {
+		w.Show()
+		w.Focus()
+	}
+}
+
+func (t *Tray) showMainAt(url string) {
+	if t.svc.WindowManager != nil {
+		t.svc.WindowManager.ShowMainAt(url)
+		return
+	}
+	if w := t.mainWindow(); w != nil {
+		w.SetURL(url)
+		w.Show()
+		w.Focus()
+	}
 }
 
 // applyLanguage re-renders every translated surface in the Localizer's current