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/KanbanApp.swift

@@ -183,6 +183,14 @@ struct KanbanApp: App {
             FindSteppingCommands()
         }
 
+        // The system's own toolbar rows — Show/Hide Toolbar and **Customize Toolbar…**, which
+        // 11-command-nexus.md files under Standard macOS furniture ("Customize Toolbar… per system
+        // convention (03)"). They are the platform's, spelled by the platform: both are nil-target
+        // AppKit actions the key window's toolbar answers, so they validate per window (disabled on
+        // welcome, live on the board and card windows) with nothing of ours in between. The
+        // right-click ▸ Customize Toolbar… path 03 names is AppKit's too, and needs no row at all.
+        ToolbarCommands()
+
         // The View menu. `CommandGroupPlacement.toolbar` *is* View — the menu the toolbar's own
         // items live in — which is where 11-command-nexus.md files Show Trash. A divider separates it
         // from the card window's three view-state rows below: one board-scoped toggle, then a