Offline banner: label the bundled-dump fallback with its snapshot date

The no-network last resort (bundled real dump) now shows a tappable banner
above the tabs — 'Offline data from 15 Aug / Pull to refresh to update
prices' — so stale snapshot data is never presented as live. Banner date
comes from the dump's own data_updated stamp (FuelStore.offlineDataLabel,
ISO 8601 with/without fractional seconds). Cleared on any successful fetch.

- BundledDumpProvider.dataUpdatedStamp (JSONSerialization, app target)
- ContentView: offlineDataDate state, offlineBanner view (tip-card styling,
  orange wifi.slash + arrow.clockwise, tap = force refresh), refresh()
  clears/sets it, -forceOfflineDump QA launch hook (serves the dump and
  skips auto-refresh so the banner stays up for captures)
- Localizable.strings: 3 keys (banner title/subtitle/a11y)
- +3 tests: OfflineDataLabelTests (101 total)

FuelBoard/BundledDumpProvider.swift

@@ -15,4 +15,14 @@ enum BundledDumpProvider {
         guard let asset = NSDataAsset(name: "FuelBoardDump") else { return nil }
         return try? FuelPriceProvider.decodeStations(from: asset.data)
     }
+
+    /// The GOV.UK `data_updated` stamp from the bundled envelope (ISO 8601).
+    /// Labels the offline-data banner honestly ("Offline data from 15 Aug")
+    /// instead of presenting the build's snapshot as live.
+    static var dataUpdatedStamp: String? {
+        guard let asset = NSDataAsset(name: "FuelBoardDump"),
+              let obj = try? JSONSerialization.jsonObject(with: asset.data)
+                  as? [String: Any] else { return nil }
+        return obj["data_updated"] as? String
+    }
 }