Implement toolbar customization for both windows

NSToolbar through the existing HostedWindowController rather than
SwiftUI's toolbar — for reasons that are contract, not taste: 03's
transient-search clause is a decision over the toolbar's current
contents, which NSToolbar publishes and SwiftUI's API cannot answer;
Undo/Redo are the system's nil-target responder-chain actions so the
toolbar items validate exactly as the menu rows do (disabled on base
boards, alive in m8 unchanged); and the search item hosts the real
NSSearchField with explicit first-responder control. Customization is
all system furniture — Customize sheet, drag rearrange, display-mode
popup, overflow, autosaved per window kind. Board default: the search
field alone, trailing; catalog adds New Card, New Lane, Undo, Redo,
Show Trash, every action extracted from its menu command so no second
predicate exists. Card default: the Edit Body / Raw Source toggles and
Add Attachment, mirroring their commands' own predicates live via
observation tracking. Removing the search item keeps the promise —
⌘F surfaces the same field as a transient strip under the title bar,
persisting until the query clears, and an overflowed item that cannot
take the keyboard falls through to the strip too.

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

Kanban/UI/Board/BoardCommands.swift

@@ -304,11 +304,10 @@ struct BoardCreationCommands: View {
 
     var body: some View {
         Button("New Card") {
-            guard let store, let target = newCardTarget else { return }
-            store.transient.beginPlaceholder(inLane: target.laneID, after: target.anchorCardID)
+            store?.beginNewCard()
         }
         .keyboardShortcut("n", modifiers: .command)
-        .disabled(newCardTarget == nil)
+        .disabled(store?.newCardTarget == nil)
 
         Button("New Lane") {
             store?.createLane()
@@ -316,17 +315,32 @@ struct BoardCreationCommands: View {
         .keyboardShortcut("n", modifiers: [.shift, .command])
         .disabled(store?.acceptsBoardMutations != true)
     }
+}
 
-    /// Where ⌘N would file a card, or `nil` when it cannot — no focused board, a board that refuses
-    /// writes, an inline editor holding the keyboard, or a board with no lanes.
-    private var newCardTarget: NewCardTarget.Resolution? {
-        guard let store, store.acceptsBoardMutations else { return nil }
+extension BoardStore {
+
+    /// Where ⌘N would file a card, or `nil` when it cannot — a board that refuses writes, an inline
+    /// editor holding the keyboard, or a board with no lanes.
+    ///
+    /// On the store rather than private to the menu row because **the toolbar's New Card item is the
+    /// same command** (03-board-ui.md ▸ Toolbar: toolbar items mirror menu commands), and a second
+    /// derivation of this rule would be a second chance to disagree with it — the same reason the row
+    /// itself uses one answer for both its action and its validation.
+    var newCardTarget: NewCardTarget.Resolution? {
+        guard acceptsBoardMutations else { return nil }
         return NewCardTarget.resolve(
-            selection: store.selection,
-            lastActiveLaneID: store.transient.lastActiveLaneID,
-            snapshot: store.snapshot
+            selection: selection,
+            lastActiveLaneID: transient.lastActiveLaneID,
+            snapshot: snapshot
         )
     }
+
+    /// ⌘N's action — opening the new-card placeholder wherever `newCardTarget` says. Shared with the
+    /// toolbar item that mirrors the row.
+    func beginNewCard() {
+        guard let target = newCardTarget else { return }
+        transient.beginPlaceholder(inLane: target.laneID, after: target.anchorCardID)
+    }
 }
 
 // MARK: - Board Info