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/Models/Models.swift

@@ -188,6 +188,17 @@ final class CachedStory {
     }
 }
 
+@Model
+final class ReadStory {
+    @Attribute(.unique) var id: String
+    var readAt: Date
+
+    init(id: String) {
+        self.id = id
+        self.readAt = Date()
+    }
+}
+
 @Model
 final class CachedArticle {
     @Attribute(.unique) var id: String