feat(music): network-adaptive prefetch depth, opt-in keep-screen-on toggle

Prefetch depth (music-player.js): 5 tracks ahead on Wi-Fi, 3 on cellular —
more runway through a screen-lock network gap when the connection is cheap
and fast, less when it's metered. navigator.connection is Chromium-only;
Firefox/Safari (where it's undefined) get the same conservative tier as an
unrecognized connection type, never assumed fast. MAX_CACHED_BLOBS raised
to 6 to hold the largest case (current + 5).

Keep-screen-on-during-audio (new user preference, off by default): a
Settings toggle, backed by a new whitelisted key on
/v1/users/me/preferences (music_keep_screen_on). music-player.js holds a
Screen Wake Lock only while a track is playing and only when the user has
opted in — unlike the video player's unconditional lock, this must not
fight the ordinary expectation (matching Spotify/Deezer) that the phone
locks on its own while listening.

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