feat(volume-osd): show what is playing

The OSD gains a track row above the volume bar when an MPRIS player is
running: album art, title, artist, and prev/play/next. With no player it
is the volume OSD it was, at the same size. A track change or a
play/pause shows the panel as well, so the row is not something you only
see by happening to touch the volume.

Hovering freezes the fade so the buttons can be clicked and leaving
restarts it, which keeps the passive behaviour intact for anyone not
reaching for the mouse. The panel still takes no keyboard focus.

playerctld proxies whichever player is active and republishes it under
its own bus name, so every player is enumerated twice. Player.qml drops
that name rather than deduplicating by identity, and controls the real
player, which works whether or not playerctld is running.

Album art is loaded with cache off and a per-track suffix on the URL.
Audacious extracts embedded art into one temp file in its cache and
rewrites that same path for each track, so the URL repeats while the
image changes, and a cached Image would keep showing the last cover.

Theme gains overlay, used by the art placeholder.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G7NRsGyF9jGfPYS4zPqpN7

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