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

Kanban/UI/Motion.swift

@@ -182,6 +182,19 @@ enum Motion {
         transientSearchAppearance(reduced: reduced).transition
     }
 
+    /// The card window's attributes sidebar, arriving or leaving — View ▸ Show Sidebar's one visible
+    /// consequence (05-card-window.md ▸ Composition ▸ The attributes sidebar). Slides from the
+    /// trailing edge, where the sidebar itself sits — `transientSearchAppearance`'s own reasoning
+    /// (the surface arrives from the edge it lives on) applied to this one's edge instead of the
+    /// toolbar's.
+    static func cardSidebarAppearance(reduced: Bool) -> Appearance {
+        reduced ? .crossfade : .slideAndFade(from: .trailing)
+    }
+
+    static func cardSidebarTransition(reduced: Bool) -> AnyTransition {
+        cardSidebarAppearance(reduced: reduced).transition
+    }
+
     // MARK: - The AppKit face
 
     /// The lane-resize window animation's duration, for `NSAnimationContext` — which takes a number