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)

Shared/FuelStore.swift

@@ -782,6 +782,27 @@ struct FuelStore {
         loadString(service: dataUpdatedKey)
     }
 
+    /// A short label for the offline-data banner: "15 Aug" from a GOV.UK
+    /// `data_updated` ISO 8601 stamp (with or without fractional seconds).
+    /// Nil when the stamp is missing or unparseable — callers then hide the
+    /// banner rather than label data with a wrong date.
+    static func offlineDataLabel(from stamp: String?) -> String? {
+        guard let stamp, !stamp.isEmpty else { return nil }
+        let withFraction = ISO8601DateFormatter()
+        withFraction.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
+        var date = withFraction.date(from: stamp)
+        if date == nil {
+            let plain = ISO8601DateFormatter()
+            plain.formatOptions = [.withInternetDateTime]
+            date = plain.date(from: stamp)
+        }
+        guard let date else { return nil }
+        let formatter = DateFormatter()
+        formatter.dateFormat = "d MMM"
+        formatter.locale = Locale(identifier: "en_GB")
+        return formatter.string(from: date)
+    }
+
     /// True when the cached data is fresh enough that a scheduled auto-refresh
     /// should be skipped (twice-a-day policy).
     static var isCacheFresh: Bool {