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/JarvisApp.swift

@@ -51,6 +51,6 @@ struct JarvisApp: App {
                 await notifications.bootstrap()
             }
         }
-        .modelContainer(for: [CachedStory.self, CachedArticle.self])
+        .modelContainer(for: [CachedStory.self, CachedArticle.self, ReadStory.self])
     }
 }