Card windows drop the title from the title bar — the body already says it

The card's name renders as part of the card body, so the chrome said it
twice. HostedWindowController gains a titleVisibility slot on the same
retained-slot pattern as the accessory and toolbar (reapplied at attach,
so the macOS 26 provisional-window swap can't lose it); CardWindowHost
opts in beside installToolbar, board windows keep AppKit's default.
window.title itself is untouched — navigationTitle still feeds the
Window menu, Exposé, VoiceOver and restoration.

Known consequence, probed empirically: under .hidden AppKit collapses
title and subtitle into one hidden field, so the "board › lane"
navigationSubtitle disappears with it — and the details sidebar carries
no placement rows, so a card window currently shows its home nowhere.
Filed on the Redesign board to bless or relocate; 05-card-window.md's
subtitle sentence needs reconciling either way.

Claude-Session: https://claude.ai/code/session_01CqjXB7ASoWtbyoGod68k97

Kanban/App/WindowAccessor.swift

@@ -91,6 +91,22 @@ final class HostedWindowController: NSObject, NSWindowDelegate {
     /// their board has loaded.
     private var toolbarController: WindowToolbarController?
 
+    /// Whether this window's title is hidden from the title bar — **card windows only**
+    /// (05-card-window.md ▸ Window: the card's name is shown as part of the card's body, not the
+    /// chrome). `nil` leaves AppKit's own default (`.visible`) untouched, which is what every board
+    /// window keeps without a call of its own — the same "nothing to do" posture `titlebarAccessory`
+    /// has on welcome, the bootstrap and card windows, mirrored here for the one window kind that
+    /// *does* have an opinion.
+    ///
+    /// A slot, not a one-shot write, for the accessory and toolbar's own reason: the value has to
+    /// survive the provisional-window swap (`detach()`'s doc comment) and reapply itself when the
+    /// real window attaches, which a write made once at `onAttach` time would not survive if that
+    /// closure only fired for the provisional window. `NSWindow.title` itself is a different slot
+    /// entirely — SwiftUI's `navigationTitle` sets it directly, and it is left alone on purpose: the
+    /// Window menu, Mission Control/Exposé, VoiceOver and state restoration all read the string, not
+    /// what the chrome draws from it.
+    private var titleVisibility: NSWindow.TitleVisibility?
+
     private static let logger = Logger(subsystem: "dev.rzen.indie.Kanban", category: "window")
 
     // MARK: Attachment
@@ -110,6 +126,7 @@ final class HostedWindowController: NSObject, NSWindowDelegate {
         // window existed is installed here instead, and one handed over later installs immediately.
         addTitlebarAccessoryIfPossible()
         applyToolbarIfPossible()
+        applyTitleVisibilityIfPossible()
     }
 
     /// Puts the previous delegate back and takes the titlebar accessory and toolbar off the window —
@@ -192,6 +209,26 @@ final class HostedWindowController: NSObject, NSWindowDelegate {
         window.toolbar = nil
     }
 
+    // MARK: Title visibility
+
+    /// Hides this window's title from the title bar, leaving the toolbar exactly as it renders today
+    /// — the card-window seam (`CardWindowHost`, 05-card-window.md ▸ Window). `window.title` is
+    /// untouched by this call on purpose; see the property's doc comment for why.
+    ///
+    /// Safe to call whenever the caller learns it wants this — before the window exists (the value is
+    /// held and applied at `attach`) or after (applied immediately) — and safe to call more than once,
+    /// unlike the accessory and toolbar slots: writing `NSWindow.titleVisibility` twice has no side
+    /// effect worth guarding against, so this is not an install-once seam.
+    func hideTitle() {
+        titleVisibility = .hidden
+        applyTitleVisibilityIfPossible()
+    }
+
+    private func applyTitleVisibilityIfPossible() {
+        guard let window, let titleVisibility else { return }
+        window.titleVisibility = titleVisibility
+    }
+
     /// Closes the window for real, after the flush has run. `performClose` rather than `close` so the
     /// standard path runs — SwiftUI's own delegate gets its callbacks, tabbing behaves — with the
     /// flag telling our own `windowShouldClose` to stand aside.