Adds a 6px col-resize strip on the right edge of the desktop sidebar.
Drag updates --sidebar-width on document.documentElement (clamped
180-480px). Mouse + touch handlers; double-click resets to the 240px
default. Width persists via localStorage["paliad-sidebar-width"], read
on every page load before first paint so layout is stable from frame 1.
The handle is opacity-faded on the icon-rail (collapsed) state and
hidden entirely under the mobile breakpoint, since the mobile sidebar
is an overlay drawer that always uses the fixed --sidebar-expanded
width.
Pin/unpin behaviour is preserved: pinned state keeps the user's chosen
width; unpinning drops to the icon rail; hover-expand restores the
chosen width. The hover-collapse mouseleave handler ignores transitions
during an active drag so the sidebar doesn't snap shut mid-resize.
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