Settings About: data source section (connection, station count, GOV.UK update time)

- Relay envelope now carries source (api|csv), stations_count, and
  data_updated (freshest price_last_updated the GOV.UK server reports,
  independent of relay sync time).
- App decodes the new envelope metadata (RelayMeta), persists it after
  each full fetch, and shows it in Settings → About → Data source:
  Connection (API/CSV), Stations count, Data updated (local-formatted).
- Older relays without the fields decode cleanly (nil meta → em-dash).
- 2 new tests for meta decode + absent-meta tolerance; 37/37 pass.

Shared/FuelStore.swift

@@ -264,6 +264,9 @@ struct FuelStore {
     static let debugModeKey = "fuelboard.debugMode"         // Bool — hidden dev flag
     static let onboardingCompletedKey = "fuelboard.onboardingCompleted" // Bool
     static let lastRefreshKey = "fuelboard.lastRefresh"      // TimeInterval (seconds since 1970)
+    static let relaySourceKey = "fuelboard.relaySource"      // String — "api" | "csv"
+    static let stationCountKey = "fuelboard.stationCount"    // String — station count
+    static let dataUpdatedKey = "fuelboard.dataUpdated"      // String — govUK dataset update time
 
     // MARK: Stations
     // The full-UK dataset (~2.9 MB) lives in app-group UserDefaults only —
@@ -503,6 +506,34 @@ struct FuelStore {
         saveString(String(date.timeIntervalSince1970), service: lastRefreshKey)
     }
 
+    // MARK: Relay metadata — shown in Settings → About. Written after each
+    // successful full fetch so the About section reflects the live source.
+
+    static func saveRelayMeta(_ meta: RelayMeta) {
+        if let source = meta.source {
+            saveString(source, service: relaySourceKey)
+        }
+        if let count = meta.stationCount {
+            saveString(String(count), service: stationCountKey)
+        }
+        if let updated = meta.dataUpdated {
+            saveString(updated, service: dataUpdatedKey)
+        }
+    }
+
+    static func loadRelaySource() -> String? {
+        loadString(service: relaySourceKey)
+    }
+
+    static func loadStationCount() -> Int? {
+        guard let raw = loadString(service: stationCountKey) else { return nil }
+        return Int(raw)
+    }
+
+    static func loadDataUpdated() -> String? {
+        loadString(service: dataUpdatedKey)
+    }
+
     /// True when the cached data is fresh enough that a scheduled auto-refresh
     /// should be skipped (twice-a-day policy).
     static var isCacheFresh: Bool {