The board wears a picture — background becomes a mapping, and the window chrome follows it under a thin frost

background is {color:, image:} and only a mapping at every level; the board's image paints the full window under a transparent title bar, with a thin-material frost strip keeping the chrome legible and the standard accommodations intact.

Claude-Session: https://claude.ai/code/session_014PtZdPwqZuqEDLc6wZMtEy

Kanban/Storage/BoardModel.swift

@@ -88,6 +88,21 @@ public struct BoardModel: Sendable, Equatable {
     public let deleted: FieldValue<Date>
 
     public let background: FieldValue<String>
+
+    /// The `background` mapping's `image` subkey — a path **relative to `rootURL`**
+    /// (01-storage-format.md § Frontmatter; 03-board-ui.md § Styling ▸ Capabilities).
+    ///
+    /// **Board-level only**, which is why `Lane` and `Card` carry no twin: a lane's and a card's
+    /// colour are edge accents, and there is nothing at those levels an image could fill. The
+    /// shared reader still accepts the mapping at every level for the colour's sake — one key, one
+    /// reading — but this half has exactly one consumer, the board window's backdrop.
+    ///
+    /// A **reading, not a location**: the path is resolved (and required to stay inside the board)
+    /// where it is drawn, `BoardBackdrop.imageURL(named:inBoardRoot:)`, so a value that leads
+    /// nowhere paints nothing and stays on disk exactly as written — the same lenient degrade an
+    /// unrecognized colour gets.
+    public let backgroundImage: FieldValue<String>
+
     public let icon: FieldValue<String>
     public let iconColor: FieldValue<String>