[client] Create GUI windows on demand and destroy them on close (#7096)

The main and Settings windows were created at startup and kept alive hidden on close, so an idle tray held two webview processes for surfaces the user may never open. Both are now built on first show and destroyed on close, which takes the idle footprint on macOS from ~160 MB to ~74 MB.

The WindowManager owns creation: it rebuilds the main window on the next show and hands out live pointers, since a stored one goes stale. Every show is deferred until the frontend reports it has rendered, so a freshly created window is never on screen empty, with a timeout so a frontend that never reports cannot strand a window hidden.

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 +248,49 @@ 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) 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()
+	}
+}
+
+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) showMainAndEmit(event string) {
+	if t.svc.WindowManager != nil {
+		t.svc.WindowManager.ShowMainAndEmit(event)
+		return
+	}
+	t.showMain()
+	t.app.Event.Emit(event)
 }
 
 // applyLanguage re-renders every translated surface in the Localizer's current
@@ -479,7 +517,8 @@ func (t *Tray) handleConnect(upItem *application.MenuItem) {
 	// NeedsLogin/SessionExpired/LoginFailed won't honor a plain Up RPC — they
 	// need the Login → WaitSSOLogin → Up sequence. Emit EventTriggerLogin so
 	// the React startLogin() (which owns the BrowserLogin popup) drives it;
-	// the hidden main webview is alive and subscribed, so only the popup shows.
+	// the WindowManager materialises a hidden main webview when none is live,
+	// so only the popup shows.
 	t.statusMu.Lock()
 	needsLogin := strings.EqualFold(t.lastStatus, services.StatusNeedsLogin) ||
 		strings.EqualFold(t.lastStatus, services.StatusSessionExpired) ||