The Background tab becomes Theme — solid colors or patterns, presets only, chevron-paged

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

Kanban/LiveStore/BoardStore.swift

@@ -2050,6 +2050,81 @@ public final class BoardStore: HealHost {
         return true
     }
 
+    // MARK: - Solid background
+
+    /// **Applies a solid colour background to this board** — the `background.color` subkey set and
+    /// the `background.image` subkey removed, in one bracket (03-board-ui.md § Styling ▸ Theme tab;
+    /// the Solid color half of `BoardThemeTabView`'s picker).
+    ///
+    /// Modeled line-for-line on `applyGeneratedBackground` **minus the file write**: there is no
+    /// picture to land, so the bracket holds a single `updateIndex` rather than an image write ahead
+    /// of one. Everything else is that gesture's, restated here rather than shared because the two
+    /// brackets differ in exactly the one place that matters (one write versus two): the same
+    /// undo-restores-both-fields shape, the same `WriteOperation.setBoardBackground`, the same
+    /// swallowed failure.
+    ///
+    /// ### `facets.png` survives on disk
+    ///
+    /// Choosing a solid colour over a generated background does **not** delete the picture the
+    /// generator wrote. Undo restores the `image` *field*, and a field cannot point an undo back at
+    /// bytes this gesture just erased — so the file has to survive for the same reason
+    /// `applyGeneratedBackground`'s own overwrite-in-place does. A board that regenerates after
+    /// choosing solid still finds `facets.png` free to overwrite in place; the file becomes an orphan
+    /// only when nothing in the frontmatter ever points at it again, which is the same quiet leftover
+    /// a hand-deleted `image:` line already leaves.
+    ///
+    /// - Parameter colorHex: the solid colour to write, `#RRGGBB` — one of the Theme tab's Solid color
+    ///   swatches, which read `FacetsRecipe.primaryColorHex` at the filters' tone/saturation level.
+    /// - Returns: whether bytes reached disk (`applyGeneratedBackground`'s same rule). Discardable:
+    ///   the picker has nothing to do with the answer.
+    @discardableResult
+    public func applySolidBackground(colorHex: String) -> Bool {
+        let root = rootURL
+        let priorImage = snapshot.backgroundImage
+        let priorColor = snapshot.background
+
+        let landed: Void? = try? performWrite { () throws(BoardWriteError) -> Void in
+            try BoardWriter.updateIndex(
+                inItemFolder: root, kind: .board, operation: .setBoardBackground
+            ) { document in
+                Self.pointSolidBackground(color: colorHex, in: &document)
+            }
+        }
+        guard landed != nil else { return false }
+
+        // restyle → prior style (13-native-undo.md ▸ Rules). The board's own stack, never a window's:
+        // there is no card here to have a session.
+        registerStep(
+            HistoryPhrase.name(.restyle, kind: .board),
+            undoExpects: [.present(root, .background(colorHex), .backgroundImage(nil))],
+            redoExpects: [.present(root, .background(priorColor.value), .backgroundImage(priorImage.value))]
+        ) { _ in
+            try BoardWriter.updateIndex(
+                inItemFolder: root, kind: .board, operation: .setBoardBackground
+            ) { document in
+                // A malformed prior reads as a removal on both subkeys, `restore(_:to:in:)`'s own
+                // rule and the one the redo expectation above is written against.
+                document.setBackgroundImage(priorImage.value)
+                Self.restore(priorColor, to: FrontmatterKeys.background, in: &document)
+            }
+        } redo: { _ in
+            try BoardWriter.updateIndex(
+                inItemFolder: root, kind: .board, operation: .setBoardBackground
+            ) { document in
+                Self.pointSolidBackground(color: colorHex, in: &document)
+            }
+        }
+        return true
+    }
+
+    /// The one subkey write and the one subkey removal, spelled once so the gesture and its redo
+    /// cannot drift apart on the order they land in — `pointBackground(at:color:in:)`'s sibling, one
+    /// image write short.
+    private static func pointSolidBackground(color: String, in document: inout FrontmatterDocument) {
+        document.setStyleValue(color, for: FrontmatterKeys.background)
+        document.setBackgroundImage(nil)
+    }
+
     /// The name a generated background is written under: **ours to overwrite**, or the next free one.
     ///
     /// `current` is what `background.image` says now. When that is already the generated name the