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)

FuelBoardTests/Tests/FuelBoardSharedTests/FuelBoardTests.swift

@@ -587,3 +587,22 @@ final class PriceDisplayTests: XCTestCase {
         XCTAssertEqual(FuelStore.priceTextSpoken(100.9), "£1.009")
     }
 }
+
+// MARK: - Offline data banner label
+
+final class OfflineDataLabelTests: XCTestCase {
+    func testOfflineDataLabelFormatsStampWithFractionalSeconds() {
+        // The bundled dump's real envelope stamp.
+        XCTAssertEqual(FuelStore.offlineDataLabel(from: "2026-08-15T08:46:33.000Z"), "15 Aug")
+    }
+
+    func testOfflineDataLabelToleratesPlainISODate() {
+        XCTAssertEqual(FuelStore.offlineDataLabel(from: "2026-08-15T08:46:33Z"), "15 Aug")
+    }
+
+    func testOfflineDataLabelNilWhenMissingOrUnparseable() {
+        XCTAssertNil(FuelStore.offlineDataLabel(from: nil))
+        XCTAssertNil(FuelStore.offlineDataLabel(from: ""))
+        XCTAssertNil(FuelStore.offlineDataLabel(from: "not-a-date"))
+    }
+}