How Best Value works: a Setup Best Value button, and a clean hand-off to Settings

The explanation sheet now carries a full-width button that closes it and the
sheet it was opened from, then lands on Settings — whose Best Value section is
the first thing in its list, so the controls are on screen on arrival. It uses
.controlSize(.large), the standard iOS button height.

Both the More info sheet and the Key sheet are presented by StationsView, and
FavouritesView presents its own More info sheet — not ContentView's monitor as
assumed — so each host closes whichever sheet it owns on the notification.
Before this, one of them stayed up over the Settings tab. The button now
announces itself half a second after its own dismissal, so the child and parent
dismissals cannot race and drop one of the two.

FuelBoard/WatchSyncManager.swift

@@ -4,6 +4,11 @@ import WatchConnectivity
 extension Notification.Name {
     static let fuelBoardWatchRefreshRequested = Notification.Name("FuelBoardWatchRefreshRequested")
     static let fuelBoardShowStationDetail = Notification.Name("FuelBoardShowStationDetail")
+    /// Posted by the "How Best Value works" sheet's Setup Best Value button, so
+    /// the app can close that sheet's stack and land on Settings — whose Best
+    /// Value section is the first thing in its list. The sheet cannot switch
+    /// tabs itself: it is presented two levels below the tab view.
+    static let fuelBoardOpenBestValueSettings = Notification.Name("FuelBoardOpenBestValueSettings")
     /// Posted when a road-distance fill finishes, so the app's rows re-read the
     /// distance they are showing — those reads happen inside the row views, so
     /// nothing else would invalidate them.