[VitzS7]

Adds Spotify's Smart Shuffle for playlists played on this computer.

Smart Shuffle is the playlist loaded through context-resolve with ?spotify-apply-lenses=enhance. Spotify then returns the playlist plus its recommendations, marks the context with enhanced_context=true and each added track with provider=enhanced_recommendation. librespot loads that URI as is, so nothing changes on the librespot side. The lens only works as a query parameter, sending it as a header to context-resolve does nothing.

• The shuffle button cycles off, shuffle, Smart Shuffle (skipped where it is not available), with a sparkle on the icon in Smart Shuffle, like the official client. The mini player and island menus get a Smart Shuffle item.
• Recommended tracks get a sparkle in the queue.
• Switching it on or off reloads the playing playlist from the playing row and position. The row is found by uid, because the two forms of the context can give a relinked track different ids, and loading by uri jumped to another track.
• The icon font gains auto_awesome only. Regenerating the whole subset also changed a few unrelated glyphs, so I left those out.

Limits:

• Playlists only. Liked Songs with the lens returns 404.
• Local playback only. I found no way to pass the lens to a remote Connect device, so remote control keeps plain shuffle, and Smart Shuffle started on another device shows as normal shuffle.
• librespot shuffles the recommendations at random, it does not use Spotify's order (shuffle.distribution in the track metadata).
• Switching the mode reloads the track, so there is a short blip, and it starts playback if it was paused.
• The mode is not remembered between sessions.

Tested on Linux with a Premium account: two playlists (220 to 286 and 30 to 45 tracks), switching on and off mid track, the queue marks. cargo test --locked -p sf-player -p md3-model -p mf-i18n and qmllint pass.