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/FuelHistory.swift

@@ -216,10 +216,18 @@ enum FuelHistoryStore {
     // MARK: Network
 
     /// The mirror pointer — used for the empty-state hint ("first snapshot
-    /// landed …"). Non-fatal: nil just means no hint.
-    static func fetchLatest(base: URL = mirrorBase,
+    /// landed …"). Non-fatal: nil just means no hint. Deliberately bypasses
+    /// the HTTP cache (reloadIgnoringLocalCacheData) so an OFFLINE fetch
+    /// really fails instead of silently re-serving a cached pointer as a
+    /// success — otherwise "no network" would look like fresh data and the
+    /// offline banner would never fire. A short timeout surfaces the failure
+    /// (and the banner) quickly on a connected-but-dead network.
+    static func fetchLatest(base: URL = mirrorBase, timeout: TimeInterval = 10,
                             session: URLSession = .shared) async -> MirrorLatest? {
-        guard let (data, response) = try? await session.data(from: latestFileURL(base: base)),
+        var req = URLRequest(url: latestFileURL(base: base))
+        req.cachePolicy = .reloadIgnoringLocalCacheData
+        req.timeoutInterval = timeout
+        guard let (data, response) = try? await session.data(for: req),
               (response as? HTTPURLResponse)?.statusCode == 200,
               let latest = try? JSONDecoder().decode(MirrorLatest.self, from: data) else {
             return nil