Previously the music icon always rendered in the in-call bar, opening a
popover with a "Keine Sounds gespeichert" empty-state. Matches Discord's
pattern better to just drop the button entirely until the user has
something to play — otherwise it reads as a broken control. Live-
subscribes to soundboardStorage so adding/removing sounds mid-call flips
the button in or out without reopening the call. Also closes the popover
automatically if the user clears their last sound while it's open.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
