Replace deeplink exp scheme handling with an environment variable

We added some code to test deeplinks with expo's recommended method, but it's actually quite limited and doesn't allow passing query parameters with a custom host (how we handle deeplinks from push notifications), so I think we should remove that code and instead add a very simple check to overwrite the current link with an env variable if it's set.

- https://github.com/getAlby/go/blob/master/lib/link.ts#L22
- https://github.com/getAlby/go/blob/master/lib/link.ts#L121-L133
- https://github.com/getAlby/go/pull/369/files#r2501862189 <-- should be loaded from an environment variable instead