The shared SidePanel now follows the nav rail's model, so Filters, Plex and
Playlists all inherit it: unpinned it sits as a small tab docked flush against
the rail (replacing the full-height vertical strip) and expands into the card on
hover or keyboard focus without pushing content; pinned it stays expanded
in-flow. The collapse toggle became pin/unpin, and panels now default to
unpinned. The tab carries the active-filter count and an accent ring.
The tab stays mounted under the open card on purpose: swapping it out from under
the pointer drops the hover that opened the card, and unmounting it while
focused strands keyboard focus.
Measured: feed content starts at 123px instead of 336px when unpinned.
peter
79d9bca290