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/FavouritesView.swift

@@ -17,6 +17,10 @@ struct FavouritesView: View {
     var onToggleFavourite: (FuelStation, FuelType) -> Void = { _, _ in }
     /// Persists a reordered favourites array (after drag-and-drop).
     var onReorder: ([FavouriteEntry]) -> Void = { _ in }
+    /// Propagated from ContentView — Trends' history failure with no data
+    /// raises the global connection banner; recovery clears it.
+    var onHistoryUnavailable: (() -> Void)? = nil
+    var onHistoryRecovered: (() -> Void)? = nil
 
     /// Fuel types that currently have at least one favourite — these are the
     /// only tabs shown (a fuel with no favourites gets no tab).
@@ -62,7 +66,9 @@ struct FavouritesView: View {
          distanceUnit: DistanceUnit,
          priceDisplayStyle: PriceDisplayStyle,
          onToggleFavourite: @escaping (FuelStation, FuelType) -> Void = { _, _ in },
-         onReorder: @escaping ([FavouriteEntry]) -> Void = { _ in }) {
+         onReorder: @escaping ([FavouriteEntry]) -> Void = { _ in },
+         onHistoryUnavailable: (() -> Void)? = nil,
+         onHistoryRecovered: (() -> Void)? = nil) {
         self.favourites = favourites
         self.selectedFuel = selectedFuel
         self.location = location
@@ -70,6 +76,8 @@ struct FavouritesView: View {
         self.priceDisplayStyle = priceDisplayStyle
         self.onToggleFavourite = onToggleFavourite
         self.onReorder = onReorder
+        self.onHistoryUnavailable = onHistoryUnavailable
+        self.onHistoryRecovered = onHistoryRecovered
         _fuel = State(initialValue: selectedFuel)
     }
 
@@ -158,7 +166,9 @@ struct FavouritesView: View {
                 TrendsView(
                     favourites: favourites,
                     selectedFuel: activeFuel,
-                    priceDisplayStyle: priceDisplayStyle
+                    priceDisplayStyle: priceDisplayStyle,
+                    onHistoryUnavailable: onHistoryUnavailable,
+                    onHistoryRecovered: onHistoryRecovered
                 )
             }
             .onAppear {