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.

FuelBoard/ContentView.swift

@@ -293,6 +293,11 @@ struct ContentView: View {
             stations = fetched
             FuelStore.saveStations(fetched)
             FuelStore.saveLastRefresh()
+            // Persist relay envelope metadata (source, station count, GOV.UK
+            // dataset update time) for the Settings → About section.
+            if let meta = RelayFuelProvider.latestMeta {
+                FuelStore.saveRelayMeta(meta)
+            }
             // Keep the keychain favourites fresh with the new prices — the
             // widget's Favourites mode reads them from keychain (the only
             // channel shared on SideStore free), so stale star-time snapshots