Build the welcome screen

The welcome window becomes the real thing: Xcode-style, hidden title
bar with background drag, branding and actions left, recents right —
rows carrying the board symbol, name, location, and the registry's
cached lane/card counts (stamped at close, never a scan at welcome
time), sorted by last opened. Launch failures surface row-level per
02: a failure joins its recents row as a warning caption, an
unresolvable bookmark renders unavailable with Forget its one
affordance, and only a failure with no row to carry it falls back to
a compact list; a board opening again heals its row. New Board
(Opt-Cmd-N) opens the Pages-style template chooser — shipped with
the single Basic template and the m9 seams marked — flowing through
the save panel into createBoard/createLane and straight into a board
window. Open Recent gains its submenu with Clear Menu (byte-identical
to forgetting every row, pinned by test), and File > Duplicate forks
the frontmost board to a Finder-style copy sibling: pending work
flushes first through the close flush's step two alone (sessions stay
open — 09's stated exception), every GUID and tombstone carries (the
whole-board carve-out from copies-remint), and the copy opens in its
own window while the original stays put. 36 new tests.

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

Kanban/App/BoardWindowHost.swift

@@ -95,6 +95,9 @@ struct BoardWindowHost: View {
             // beside it the window's own popover flag, which is what File ▸ Board Info toggles, and
             // its purge-alert host, which the trash's two confirmed commands raise.
             .focusedSceneValue(\.boardStore, store)
+            // The window's identity beside its store — File ▸ Duplicate flushes a *session*, which
+            // is keyed on the window rather than on the board it is showing.
+            .focusedSceneValue(\.boardWindowRef, ref)
             .focusedSceneValue(\.boardInfo, boardInfo)
             .focusedSceneValue(\.trashConfirmations, trashConfirmations)
         }