WebView: Make preload a string.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

app/renderer/js/main.ts

@@ -389,7 +389,7 @@ export class ServerManagerView {
           },
           onTitleChange: this.updateBadge.bind(this),
           nodeIntegration: false,
-          preload: true,
+          preload: "js/preload.js",
         }),
       }),
     );