Build the card window shell and lifecycle

The m4 scene plumbing was already honest — one WindowGroup value per
CardWindowRef enforces one-window-per-card, and CardWindowFate's
ancestor walk answered dismissal — so this card fills the window: a
two-column shell whose body column takes all resize flex and whose
sidebar width derives once from font metrics (26 characters of average
body advance plus em gutters), the five 05-ordered section headers as
placeholders, and the card body as selectable plain text until Preview
mode lands. The fate walk now returns a CardPlacement (card + lane), so
one pass answers both liveness and the live board › lane subtitle; a
board rename lands for free through displayName. Card windows remember
their frames per card in the board record (case-folded id keys,
unchanged-writes-nothing), restoring instead of cascading; only
unremembered cards take the last-used size and cascade. Store
acquisition stays gated on liveStore — a card window never opens a
board — and the close-flush hook stands with nothing to flush until the
Edit-session card.

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

Kanban/KanbanApp.swift

@@ -113,6 +113,11 @@ struct KanbanApp: App {
         }
         .restorationBehavior(.disabled)
         .defaultLaunchBehavior(.suppressed)
+        // The **first** card window's size, and only that one: every later window opens at the
+        // last-used size or at its card's remembered frame, both applied by the host as the window
+        // attaches (05-card-window.md ▸ Window). Derived from font metrics like every other
+        // measurement in that window rather than written down in points.
+        .defaultSize(CardWindowMetrics.defaultSize(bodyPointSize: CardWindowMetrics.bodyPointSize))
 
         Settings {
             SettingsView()