Default route cannot be set to something unknown to app.routes

The Frontend logic disallows setting a default_route value that is unknown to the `app` as it finds the path within `app.routes`. See:

```js
const defaultRoute = app.forum.attribute('defaultRoute');
let defaultAction = 'index';

for (const i in app.routes) {
if (app.routes[i].path === defaultRoute) defaultAction = i;
}

app.routes[defaultAction].path = '/';
```

It would be great to allow users and extensions to override the value so it actually works, eg pointing to a specific discussion or tag.

Start by tracing the frontend logic that reads `defaultRoute` and searches `app.routes`, using the shown route initialization as the entry point. Determine how an unknown route can be preserved or overridden by users and extensions, then verify that `default_route` can target a specific discussion or tag instead of only a registered route.

Written by the indexing model from the issue text.