Add specs for the account menu, downloads-as-offline-library, offline
downloaded-only filter, and scoped search (+ boundary revision). Add the
new UR/DR entries to requirements.md, update ux-flows, and regenerate the
traceability matrix.
TRACES: UR-049, UR-050, UR-052, UR-053, UR-054, UR-055, UR-056
Hand video playback off to a native audio-only stream when the app is
backgrounded or locked, with no on-device video decode (UR-040). Adds
player_enter/exit_background_audio commands, an audio-only stream URL
for video items across the repository layer, and the frontend handoff
state machine wired into VideoPlayer. Includes accompanying
repository/offline/player refactoring and regenerates the traceability
matrix.