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/CardWindowShellTests.swift

@@ -178,6 +178,33 @@ struct CardWindowMetricsTests {
         #expect(CardWindowMetrics.minimumSize(bodyPointSize: size).height > 0)
     }
 
+    @Test("Hiding the sidebar shrinks the window's minimum by exactly its width")
+    func hidingTheSidebarShrinksTheMinimum() {
+        // View ▸ Show Sidebar's own mirror of the comments column's own rule
+        // (`CommentsWindowMinimumTests.theMinimumGrowsOnlyBeside`): a pane costs the window no width
+        // while it is not shown.
+        let size: CGFloat = 13
+        let shown = CardWindowMetrics.minimumSize(bodyPointSize: size)
+        let hidden = CardWindowMetrics.minimumSize(bodyPointSize: size, sidebar: false)
+
+        #expect(shown.width == hidden.width + CardWindowMetrics.sidebarWidth(bodyPointSize: size))
+        #expect(hidden.width == CardWindowMetrics.bodyMinimumWidth(bodyPointSize: size))
+        #expect(hidden.height == shown.height, "hiding the sidebar costs width, never height")
+    }
+
+    @Test("Hiding the sidebar composes with the comments column's own minimum")
+    func hidingTheSidebarComposesWithTheCommentsColumn() {
+        let size: CGFloat = 13
+        let minimum = CardWindowMetrics.minimumSize(bodyPointSize: size, sidebar: false, commentsColumn: true)
+
+        #expect(
+            minimum.width
+                == CardWindowMetrics.bodyMinimumWidth(bodyPointSize: size)
+                    + CardWindowMetrics.commentsMinimumWidth(bodyPointSize: size),
+            "sidebar hidden and comments beside: the minimum is the body's floor plus the comments floor, nothing else"
+        )
+    }
+
     @Test("A first card window opens larger than the minimum")
     func theDefaultSizeIsRoomier() {
         let size: CGFloat = 13