banner: top-floating; fix offline-trigger cache; onboarding prompt timing

- Banner floats at the very top (over the nav/title area), never pushing the
  content below and staying clear of the list/pill.
- Offline banner now fires on refresh in airplane mode / no data: the mirror
  live chain bypassed the HTTP cache (+10s timeout) so an offline fetch really
  fails instead of silently re-serving a cached pointer+dump as a 'success'
  (which kept dataStatus .live and hid the banner).
- Onboarding permissions fire at the Continue/Allow tap BEFORE advancing, so
  the system prompt appears after the page is read and never covers the next
  page's animation (grant auto-advances).

Shared/MirrorFuelProvider.swift

@@ -96,9 +96,13 @@ struct MirrorFuelProvider: FuelPriceProviding {
         if Self.canReuseCache(cachedDay: cache?.day, latestDay: day), let cached = cache?.data {
             data = cached
         } else {
-            let (fetched, response) = try await URLSession.shared.data(
-                from: FuelHistoryStore.historyFileURL(day: day, base: baseURL)
-            )
+            // Bypass the HTTP cache + short timeout: an offline refresh must
+            // FAIL (→ offline banner), never re-serve a stale cached dump as
+            // a "successful" live fetch.
+            var req = URLRequest(url: FuelHistoryStore.historyFileURL(day: day, base: baseURL))
+            req.cachePolicy = .reloadIgnoringLocalCacheData
+            req.timeoutInterval = 10
+            let (fetched, response) = try await URLSession.shared.data(for: req)
             guard (response as? HTTPURLResponse)?.statusCode == 200 else {
                 throw FuelProviderError.mirrorUnavailable
             }