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/App/AppModel.swift

@@ -127,6 +127,41 @@ public enum AppPreferences {
     /// `double(forKey:)` reading safe here in a way `boardZoomLevel`'s note warns it usually is not.
     public static let commentsColumnWidthKey = "commentsColumnWidth"
 
+    // MARK: The attributes sidebar's visibility
+
+    /// **View ▸ Show Sidebar** — the card window's trailing attributes sidebar (05-card-window.md ▸
+    /// Composition), `showComments`'s shape and reasons exactly: one bit, app-wide, persisted across
+    /// restarts, and the checkmark (and the toolbar toggle that mirrors it, `CardToolbar`) reads
+    /// exactly this value so neither ever lies.
+    ///
+    /// **Default on.** The sidebar has been part of every card window since m6; an unset key must
+    /// read as the window everyone already knows rather than as a surprise collapse the first time
+    /// this preference exists to read.
+    public static let showCardSidebarKey = "showCardSidebar"
+
+    /// Read outside a view — `CardToolbar`'s toggle item, which is AppKit rather than SwiftUI and so
+    /// has no `@AppStorage` of its own (`lastCardWindowSize`'s reason).
+    public static var showCardSidebar: Bool {
+        UserDefaults.standard.object(forKey: showCardSidebarKey) as? Bool ?? true
+    }
+
+    /// Show/Hide Sidebar's whole behaviour — **the one write path both of its faces share**
+    /// (`BoardStore.setTrashVisible`'s precedent: "a toolbar item is a menu command with a different
+    /// face, never a second implementation of it"), so `ShowSidebarCommand`'s Toggle and
+    /// `CardToolbar`'s item call this rather than writing `UserDefaults` each in their own words.
+    ///
+    /// **A user-initiated structural change**, so it animates in the structural voice and goes
+    /// instant under Reduce Motion — the Show Trash re-divide's own reasoning, applied to a pane
+    /// leaving a window instead of a lane leaving a strip. Reduce Motion is read from AppKit rather
+    /// than from `@Environment` for the same reason `setTrashVisible` reads it that way: a toolbar
+    /// item's action and a menu row's both run outside any rendered hierarchy.
+    @MainActor
+    public static func setShowCardSidebar(_ shown: Bool) {
+        withAnimation(Motion.structural(reduced: Motion.prefersReducedMotion)) {
+            UserDefaults.standard.set(shown, forKey: showCardSidebarKey)
+        }
+    }
+
     /// The quick-style row's recently-used backgrounds — an array of palette names / hex strings,
     /// most-recent-first (03-board-ui.md § Styling ▸ Controls: "Recents are app-wide and persist
     /// app-side (user preference, never board data)"; 11-command-nexus.md files it under the