OAuth Authorization Code flow, with tokens kept server-side: refreshing needs
the client secret, so the browser never holds a Spotify token — it asks this
instance what is playing and this instance calls Spotify.
The callback arrives as a plain browser redirect with no Authorization header,
so the OAuth state carries the user id signed with the instance secret and is
compared in constant time; without that, anyone could bind their Spotify
account to another user.
Activities are now tracked per producer. pushActivities replaced the whole
list, so the desktop game detector and Spotify would erase each other — losing
exactly the case this is for, a game and Spotify at once.
Polling backs off when the tab is hidden and keeps the last known track on a
network error rather than reporting 'stopped listening'. A rejected refresh
token (access revoked on Spotify's side) drops the row so the UI stops
claiming a live connection.
Scope is read-only: user-read-currently-playing and user-read-playback-state.
Per the fork's language rule, the new UI ships in en and pt-BR, and this
round also translates the privacy panel.
