music: play, play next, add to queue

The player's queue could only be replaced: every onPlayQueue reset
tracks/order/pos together. It becomes one reducer (queue-ops.js) with
an `op`, because two appends batched into one tick cannot both read the
track count out of separate useStates.

A shared pop-up menu (menu.js) carries the three verbs, on right-click
and on a dots button. A track row is now a div holding two buttons: a
button cannot contain a button.

Found by the browser probe: both music wrappers took two arguments and
forwarded two, so every "add to queue" arrived as a plain play.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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