Private session toggle in the TUI and GUI settings #850

What

A user on Discord asked for a private session, like the one in the Spotify app. During a private session, Spotify does not share your listening activity with followers. The plays also do not change your recommendations or your taste profile. spotatui has no private session. Everything you play on the native streaming device counts against your profile.

The toggle should behave like Spotify's:

• A "Private session" row in the TUI settings screen and on the GUI Settings page.
• A lock icon in the TUI playbar and the GUI player bar during a private session.
• The private session does not survive a restart. It is runtime state only and never goes into config.yml or state.yml.

Where to look

• The librespot fork: connect/src/state.rs builds the DeviceInfo once in ConnectState::new and never sets is_private_session (connect.proto, field 11). The fork needs a way to change it at runtime and publish the new state. LegacyEndSong.proto and RawCoreStream.proto also have an incognito_mode field.
• The settings: src/core/app/settings_schema.rs and settings_apply.rs for the row, with a setting_requirement arm for Requirement::SpotifySession.
• The GUI: gui/src/Settings.tsx and gui/src/PlayerBar.tsx.
• The TUI playbar: src/tui/ui/player.rs.
• The device list: src/infra/spotify/player.rs already reads is_private_session from other Connect devices.

Possible solution

Add a setter in the fork that changes is_private_session on the device and sends a new PutStateRequest. Then add an Action in spotatui that calls the setter, and show the state on both frontends.

Before you write the fork part, capture what the Spotify desktop app sends when you toggle private session. The device flag alone is possibly not enough.

Also decide if a private session pauses spotatui's own listening history (Stats, streaks, recap) and Discord Rich Presence. Spotify hides your activity from friends during a private session, so a hidden Discord presence matches that.

Done when

• You can turn private session on and off in the TUI settings and in the GUI settings.
• During a private session, the official Spotify app shows the spotatui device as private.
• Plays during a private session do not appear in "Recently played".
• A restart of spotatui ends the private session.