Feed resilience, clearer empty states, and read/unread

- Don't blank the feed on refresh: keep current stories until new data
  arrives, and ignore benign URLError.cancelled (-999) so a network blip
  no longer wipes the list. Coalesce WebSocket story.created bursts into a
  single debounced refresh.
- Empty state now distinguishes polling, couldn't-reach-server (with the
  error + host + Retry), offline, and genuinely-empty.
- Track read stories in SwiftData (ReadStory); mark a story read when opened
  and drop/dim its signal stripe across the feed and Latest/Saved/Search.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Jarvis/Networking/APIClient.swift

@@ -19,6 +19,13 @@ enum APIError: Error, LocalizedError {
         case .networkError(let e):  return e.localizedDescription
         }
     }
+
+    /// A benign URLSession cancellation (-999), e.g. a request superseded or
+    /// dropped on a network-path change. Should not be shown or wipe state.
+    var isCancelled: Bool {
+        if case .networkError(let e) = self { return (e as? URLError)?.code == .cancelled }
+        return false
+    }
 }
 
 private struct APIErrorResponse: Decodable {