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/ContentView.swift

@@ -865,6 +865,15 @@ struct ContentView: View {
                 .onReceive(NotificationCenter.default.publisher(for: .fuelBoardRoadDistanceDebugUpdated)) { _ in
                     roadDistanceDebugStatus = RoadDistanceService.currentDebugStatus()
                 }
+                .onReceive(NotificationCenter.default.publisher(for: .fuelBoardOpenBestValueSettings)) { _ in
+                    // "Setup Best Value" in the How Best Value works sheet: close
+                    // the More info sheet it was opened from (nil dismisses it —
+                    // the sheet is item-driven) and switch to Settings. Tag 3 is
+                    // the Settings tab; its Best Value section is the first item
+                    // in the list, so the controls are on screen on arrival.
+                    monitor.pendingDetailStation = nil
+                    selectedTab = 3
+                }
                 .onReceive(NotificationCenter.default.publisher(for: .fuelBoardShowStationDetail)) { note in
                     let info = note.userInfo ?? [:]
                     if let id = info["stationID"] as? String,