Build the styling system and shared style editor

One style-editor component, anchor-agnostic: a background grid (None
well plus the 12 palette colors) and a curated symbol grid (the
pathfinder's five-dozen set, leading well removing the icon key for
the level default), selection-aware across cards, lanes, and the
board itself. Batch edits compute per-dimension state — uniform,
mixed (no well selected), or an off-palette value labeled verbatim
outside the grids — and choosing a well applies to the whole target
set as one write bracket, skipping no-ops per field. The popover
tracks its target set live per the freshly ratified rule: targets
re-resolve by UUID on every reload, a vanished target leaves the set,
an emptied set dismisses the editor, and nothing ever silently
retargets to the board. Anchors landing now: Board > Style
(Opt-Cmd-S) and the card/lane context menus, which also carry the
quick-style recents row (app-wide, persisted, capped at six, None
never recorded) and the lane's width control twinning the menu
chords. The styling system's other two renders arrive with it: a
lane's background paints the C7 top-edge band, the board's paints
the window content background — malformed values paint nothing and
stay byte-identical on disk. 31 new tests.

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

Kanban/App/AppModel.swift

@@ -50,6 +50,13 @@ public enum AppPreferences {
     public static func setLastCardWindowSize(_ size: CGSize) {
         UserDefaults.standard.set(NSStringFromSize(size), forKey: lastCardWindowSizeKey)
     }
+
+    /// The quick-style row's recently-used backgrounds — an array of palette names / hex strings,
+    /// most-recent-first (03-board-ui.md § Styling ▸ Controls: "Recents are app-wide and persist
+    /// app-side (user preference, never board data)"; 11-command-nexus.md files it under the
+    /// preferences that "need no UI"). Read and written by `StyleRecents`, which owns the list rule;
+    /// the key is declared here with its neighbours for `WindowID`'s reason.
+    public static let quickStyleBackgroundsKey = "quickStyleBackgrounds"
 }
 
 // MARK: - Launch failures
@@ -147,6 +154,12 @@ public final class AppModel {
     public let storeRegistry = BoardStoreRegistry()
     public let boardRegistry: BoardRegistry
 
+    /// The quick-style row's app-wide recents (03-board-ui.md § Styling ▸ Controls). Owned here for
+    /// the registries' reason — app-scoped, and a test holds its own rather than colliding with the
+    /// app's — and reached by the context menus through the environment, since a `BoardStore` is
+    /// board-scoped and this list deliberately is not.
+    public let styleRecents = StyleRecents()
+
     // MARK: Sessions
 
     /// One open board window and everything hanging off it.