The Background tab fills in — facets rendered to order, eight hues in a carousel

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

Kanban/Storage/BackgroundField.swift

@@ -59,12 +59,35 @@ extension FrontmatterDocument {
             }
             return
         }
+        setBackgroundSubkey(FrontmatterKeys.Background.color, to: value)
+    }
+
+    /// **Writes the `image` subkey** — `setStyleValue`'s colour write, one subkey over
+    /// (03-board-ui.md § Styling ▸ Capabilities).
+    ///
+    /// The doc comment above says "There is no image picker and none is planned"; that sentence held
+    /// until generated backgrounds (DESIGN/explorations/board-backgrounds.md), which do not make one
+    /// either. What the generator writes is a *file it just created in the board folder* and the name
+    /// it wrote it under — the app is not browsing the user's pictures, it is naming its own output —
+    /// so the hand-written path stays the escape hatch it always was, and this write preserves it the
+    /// same way the colour write preserves an image: by subkey.
+    ///
+    /// Everything else is `setStyleValue`'s, deliberately shared rather than restated: the same
+    /// in-place merge, the same flow-mapping emission, the same removal-empties-the-key rule, and the
+    /// same replacement of a non-mapping shape the schema could never read.
+    public mutating func setBackgroundImage(_ value: String?) {
+        setBackgroundSubkey(FrontmatterKeys.Background.image, to: value)
+    }
+
+    /// The one merge both subkey writes go through — see `setStyleValue` for every rule it applies.
+    private mutating func setBackgroundSubkey(_ subkey: String, to value: String?) {
+        let key = FrontmatterKeys.background
         // Only a mapping has subkeys worth carrying; every other shape — absent, the retired scalar,
         // a sequence — starts empty and is replaced outright by what the app writes.
         var existing: [YAMLValue.Pair] = []
         if case let .mapping(pairs)? = self.value(for: key) { existing = pairs }
 
-        let merged = Self.merged(existing, subkey: FrontmatterKeys.Background.color, value: value)
+        let merged = Self.merged(existing, subkey: subkey, value: value)
         if merged.isEmpty {
             remove(key)
         } else {