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/ContentView.swift

@@ -284,7 +284,7 @@ struct ContentView: View {
                 updateLiveActivity()
                 // Refresh only when the cache is stale (twice-a-day policy).
                 // Skipped under the force-* hooks so the banner stays up.
-                if !args.contains("-forceOfflineDump") && !args.contains("-forceConnectionProblem") {
+                if !args.contains("-forceOfflineDump") && !args.contains("-forceConnectionProblem") && !args.contains("-forceHistoryFailure") {
                     Task { await refresh() }
                 }
             } else {
@@ -526,7 +526,9 @@ struct ContentView: View {
             distanceUnit: distanceUnit,
             priceDisplayStyle: priceDisplayStyle,
             onToggleFavourite: toggleFavourite,
-            onReorder: reorderFavourites
+            onReorder: reorderFavourites,
+            onHistoryUnavailable: { if dataStatus == .live { dataStatus = .connectionProblem } },
+            onHistoryRecovered: { if dataStatus == .connectionProblem { dataStatus = .live } }
         )
     }