Trends: price-history fetch failure with no data raises the connection banner

A history fetch that comes back empty BECAUSE the mirror is unreachable
(pointer probe failed) was only an in-sheet retry card — no global signal.
Now it also raises the red connection banner ('Check your internet
connection / Tap to try again'), so a network problem is visible on every
tab, not just inside the Trends sheet. A successful load clears the
banner (only when the banner is the connection banner — never clobbers
the offline-dump banner).

- TrendsView: onHistoryUnavailable/onHistoryRecovered closures fired from
  load() (loadFailed -> unavailable; hasAnyData -> recovered);
  -forceHistoryFailure QA hook (forces the unreachable state, skips
  auto-refresh like the other force-* hooks)
- FavouritesView: closures threaded through the Trends sheet init
- ContentView: wires them to dataStatus (.live -> .connectionProblem on
  failure; recovered clears only .connectionProblem)
- fuelboard-development skill: hook + wiring documented

FuelBoard/TrendsView.swift

@@ -16,6 +16,12 @@ struct TrendsView: View {
     let selectedFuel: FuelType
     let priceDisplayStyle: PriceDisplayStyle
 
+    /// Propagated up to ContentView so a price-history fetch that fails with
+    /// NO data raises the global connection banner (same red banner as the
+    /// stations fetch). `onHistoryRecovered` fires once data loads again.
+    var onHistoryUnavailable: (() -> Void)? = nil
+    var onHistoryRecovered: (() -> Void)? = nil
+
     @Environment(\.dismiss) private var dismiss
 
     @State private var fuel: FuelType = .e10
@@ -80,6 +86,15 @@ struct TrendsView: View {
     }
 
     private func load() async {
+        // QA hook: force the unreachable state for screenshots (same pattern
+        // as -showTrends / -forceConnectionProblem). Runs before the fetch so
+        // the retry state renders immediately with no spinner flash.
+        if ProcessInfo.processInfo.arguments.contains("-forceHistoryFailure") {
+            series = []
+            loadFailed = true
+            onHistoryUnavailable?()
+            return
+        }
         isLoading = true
         loadFailed = false
         defer { isLoading = false }
@@ -98,6 +113,14 @@ struct TrendsView: View {
             loadFailed = firstSnapshot == nil
         }
         series = fetched
+        // A failure with no data IS a connection problem — raise the global
+        // banner so the user isn't stuck with a silent retry state. Success
+        // clears it (only if the banner is the connection banner).
+        if loadFailed {
+            onHistoryUnavailable?()
+        } else if hasAnyData {
+            onHistoryRecovered?()
+        }
     }
 
     private func yLabel(_ pence: Double) -> String {