feat: station tap action toggle (open map vs more info)

Settings → Units gains a Tap station picker: Open map (default,
historic behaviour — tap opens Apple Maps directions) or More info
(tap shows a detail sheet with name, address, distance from current
location, and a Directions button). Applies to Stations and
Favourites tabs; persisted keychain-first as
fuelboard.stationTapAction. Stations footer hint follows the setting.

Shared/FuelStore.swift

@@ -208,6 +208,25 @@ enum PriceDisplayStyle: String, Codable, CaseIterable, Identifiable {
     }
 }
 
+// MARK: - Station tap action
+
+/// What happens when a station row is tapped in the Stations/Favourites tabs:
+/// open Apple Maps directions immediately (historic behaviour, default), or
+/// show a detail sheet (name, address, distance, Directions button).
+enum StationTapAction: String, Codable, CaseIterable, Identifiable {
+    case openMap    // tap → Apple Maps directions straight away
+    case showDetails // tap → detail sheet, Directions button inside
+
+    var id: String { rawValue }
+
+    var displayName: String {
+        switch self {
+        case .openMap: return "Open map"
+        case .showDetails: return "More info"
+        }
+    }
+}
+
 // MARK: - Station model
 
 struct FuelStation: Identifiable, Codable, Equatable {
@@ -542,6 +561,22 @@ struct FuelStore {
         saveString(style.rawValue, service: priceDisplayStyleKey)
     }
 
+    // MARK: Station tap action — open map vs detail sheet. Stored raw value;
+    // default open map preserves the historic tap behaviour for existing installs.
+
+    static let stationTapActionKey = "fuelboard.stationTapAction"
+
+    static func loadStationTapAction() -> StationTapAction {
+        if let raw = loadString(service: stationTapActionKey), let action = StationTapAction(rawValue: raw) {
+            return action
+        }
+        return .openMap
+    }
+
+    static func saveStationTapAction(_ action: StationTapAction) {
+        saveString(action.rawValue, service: stationTapActionKey)
+    }
+
     /// Superscript digit glyphs for the pounds & pence format's small raised
     /// third digit (the forecourt style: "£1.29⁹").
     private static let superscriptDigits: [Character] = ["⁰", "¹", "²", "³", "⁴", "⁵", "⁶", "⁷", "⁸", "⁹"]