OAuth callback port conflicts with common services on 8888

Is your feature request related to a problem? Please describe. The Spotify OAuth callback port is hardcoded to 8888 in multiple locations across psst-core/src/oauth.rs and psst-gui/src/ui/preferences.rs. This causes a port conflict when another service is already listening on that port, making Psst's authentication fail or the other service unavailable.

Describe the solution you'd like Use a single shared constant for the OAuth redirect port instead of scattering 8888 across different files. Change the default to a less common port like 8889. This way there's one place to update if the port ever needs to change again.

Additional context The port 8888 appears in at least two files: psst-core/src/oauth.rs, psst-gui/src/ui/preferences.rs (and also README)