Pinning 2 of 3 panels previously showed a 3rd "leftover" tab-column, which
read as all-three-pinned and was confusing. Now the split view shows exactly
the pinned panels (pin 2 -> 2 columns, pin 3 -> 3). Adds an always-available
View > Side-by-side panels submenu of checkable toggles as the way to pin a
panel while already in split view (the right-click-tab gesture only works in
tabbed mode). Tests assert exactly-N-columns and the menu-pin path; the win
fixture now resets deck state so tests don't depend on persisted layout.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Pin deck panels (Export/Crop/Scan) side-by-side as resizable columns,
mirroring the playlist pin pattern; unpinned panels stay reachable as a
tab-column. Spec for the multi-pane feature.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Ethanfel