A card window toolbar control shows and hides the trailing sidebar — View ▸ Show Sidebar joins Edit Body, Raw Source and Add Attachment

The card window's trailing attributes sidebar has always been unconditional
since m6; this adds View ▸ Show Sidebar (a checkmark toggle, ShowComments'
shape) and a matching toolbar item on the customizable card toolbar
(NSToolbar/WindowToolbarController), a fourth default beside Edit Body,
Raw Source and Add Attachment. sidebar.right for the trailing pane; one
shared animated write path (AppPreferences.setShowCardSidebar) both faces
call, structural-voice reflow with a trailing slide-and-fade transition
(Motion.cardSidebarTransition), Reduce Motion respected throughout.

CardWindowMetrics.minimumSize gains a sidebar: Bool = true parameter so a
hidden sidebar shrinks the window's floor, composing with the existing
commentsColumn parameter. The toolbar item's read/write are injectable
closures (defaulted to the real UserDefaults-backed pair) so its plumbing
is testable without touching the developer's own preferences domain.
WindowToolbarController's observation tracking only sees @Observable
reads, so a small HostedWindowController.revalidateToolbar() plus an
onChange nudge keeps the toolbar button's on-state in step with the
View-menu row's write.

Scope held narrowly to visibility, per the card: no sidebar section
reordering, no action-moving.

Claude-Session: https://claude.ai/code/session_014PtZdPwqZuqEDLc6wZMtEy

KanbanTests/MotionTests.swift

@@ -145,6 +145,14 @@ struct ReduceMotionVariantTests {
         #expect(Motion.cardAppearance(reduced: true) == .crossfade)
         #expect(Motion.laneAppearance(reduced: true) == .crossfade)
     }
+
+    /// The card window's sidebar slides from the trailing edge it lives on — View ▸ Show Sidebar's
+    /// one visible consequence — and, like every slide-and-fade here, crossfades instead under
+    /// Reduce Motion.
+    @Test func theSidebarSlidesFromTheTrailingEdgeItLivesOn() {
+        #expect(Motion.cardSidebarAppearance(reduced: false) == .slideAndFade(from: .trailing))
+        #expect(Motion.cardSidebarAppearance(reduced: true) == .crossfade)
+    }
 }
 
 // MARK: - The create handoff