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/Motion.swift

@@ -132,12 +132,15 @@ enum Motion {
     enum Appearance: Equatable {
         /// Scales up from `from` while fading in (and the reverse on the way out).
         case scaleAndFade(from: CGFloat)
+        /// Slides in from `edge` while fading in, and leaves the way it came.
+        case slideAndFade(from: Edge)
         /// Opacity only — 10-accessibility.md's "crossfade" variant.
         case crossfade
 
         var transition: AnyTransition {
             switch self {
             case let .scaleAndFade(scale): .scale(scale: scale).combined(with: .opacity)
+            case let .slideAndFade(edge): .move(edge: edge).combined(with: .opacity)
             case .crossfade: .opacity
             }
         }
@@ -165,6 +168,20 @@ enum Motion {
         laneAppearance(reduced: reduced).transition
     }
 
+    /// The transient search bar arriving and leaving — ⌘F's fallback when the search field has been
+    /// removed from the toolbar (03-board-ui.md ▸ Toolbar: "with the field removed from the toolbar,
+    /// invoking it surfaces the field transiently until the search clears").
+    ///
+    /// It comes from the top edge because that is where the field lives when it is installed: the
+    /// bar is the toolbar's item arriving one row lower, not a new kind of surface.
+    static func transientSearchAppearance(reduced: Bool) -> Appearance {
+        reduced ? .crossfade : .slideAndFade(from: .top)
+    }
+
+    static func transientSearchTransition(reduced: Bool) -> AnyTransition {
+        transientSearchAppearance(reduced: reduced).transition
+    }
+
     // MARK: - The AppKit face
 
     /// The lane-resize window animation's duration, for `NSAnimationContext` — which takes a number