Add ambient reading soundscapes

[mokhov]

Problem

Some readers use background sound to focus, but doing so requires a separate player that is disconnected from reading state.

Proposed behavior

Offer a small optional collection of local, looping soundscapes controlled from the reading interface.

Acceptance criteria

• Audio never starts automatically.
• Readers can choose a sound, control volume, pause, and stop it quickly.
• Loops are seamless and bundled assets have a documented license and bounded size.
• Starting text-to-speech pauses or ducks the soundscape predictably.
• The feature can be removed from the reading controls entirely through settings.