Give the trash column a full-height marquee backdrop

The trashed-side gesture surface was sized to the rows' fitted height, so
blank column space below the last row couldn't arm a marquee — a dead
zone the board side doesn't have (DESIGN/04 > The trash: the column's
gesture surface is full height). The rows content now takes
maxHeight: .infinity like LaneView's scrollableCards, the identical
structure in the identical position, so the band arms from anywhere in
the column.

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

Kanban/UI/Board/TrashLaneView.swift

@@ -188,12 +188,21 @@ struct TrashLaneView: View {
                     .id(entry.id)
                 }
             }
-            .frame(maxWidth: .infinity, alignment: .topLeading)
+            // `maxHeight: .infinity` here, not just `maxWidth`, is what makes the gesture surface
+            // below reach the column's full height rather than stopping where the last row ends —
+            // the same fix `LaneView.scrollableCards` applies to its masonry, and for the identical
+            // reason: a `ScrollView` proposes its content only the height that content asks for, so a
+            // view sized to fit its rows leaves the blank space beneath them un-hit-testable. "The
+            // column's gesture surface is full height" (04-interactions.md ▸ The trash, settled)
+            // needs that blank space to actually belong to the view the gesture below is on.
+            .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: .topLeading)
             .padding(6)
             .contentShape(Rectangle())
-            // The band's trash-side surface. It only ever arms from the column's empty space — a
+            // The band's trash-side surface. It arms from the column's empty space, full height
-            // drag begun on a row is that row's drag-out — and the begin guard makes that geometric
+            // (above) included, so a drag can start from the blank area below the last row exactly
-            // rather than a matter of gesture priority (`MarqueeControl`).
+            // as the board background allows on the live side — a drag begun on a row instead is that
+            // row's drag-out, and the begin guard makes that geometric rather than a matter of
+            // gesture priority (`MarqueeControl`).
             .simultaneousGesture(marquee.gesture(side: .trashed))
         }
     }