Desktop OIDC: open in system browser via window.open, revert to copy-paste

The local HTTP server approach (fetch to 127.0.0.1) doesn't work in the
Wails webview. Simplify: use window.open() to launch OIDC in the real
browser. After authentication, the callback page at the server shows the
token. User copies and pastes into the Token tab.

Also fix: SetSize before app.Run() crashes with nil pointer — use
WebviewWindowOptions width/height directly from restored state.

cmd/desktop/main.go

@@ -558,20 +558,22 @@ func main() {
 
 	ws := loadWindowState()
 	width, height := 1400, 900
-	minWidth, minHeight := 1024, 700
+	if ws != nil {
+		width = ws.Width
+		height = ws.Height
+	}
 
 	window := app.Window.NewWithOptions(application.WebviewWindowOptions{
 		Title:    "Oikos",
 		Width:    width,
 		Height:   height,
-		MinWidth: minWidth,
-		MinHeight: minHeight,
+		MinWidth: 1024,
+		MinHeight: 700,
 		URL:      "/?desktop=1",
 	})
 
 	if ws != nil {
 		window.SetPosition(ws.X, ws.Y)
-		window.SetSize(ws.Width, ws.Height)
 	} else {
 		window.Center()
 	}