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/Views/Story/StoryDetailView.swift

@@ -30,6 +30,7 @@ struct StoryDetailView: View {
     let story: StorySummary
 
     @Environment(\.dismiss) private var dismiss
+    @Environment(\.modelContext) private var modelContext
     @StateObject private var vm = StoryDetailViewModel()
     @Query private var cachedArticles: [CachedArticle]
 
@@ -102,7 +103,23 @@ struct StoryDetailView: View {
         .toolbarBackground(Color.black, for: .navigationBar)
         .toolbarBackground(.visible, for: .navigationBar)
         .preferredColorScheme(.dark)
-        .task { await vm.load(id: story.id) }
+        .task {
+            markRead()
+            await vm.load(id: story.id)
+        }
+    }
+
+    // MARK: - Read state
+
+    private func markRead() {
+        let id = story.id
+        let existing = try? modelContext.fetch(
+            FetchDescriptor<ReadStory>(predicate: #Predicate { $0.id == id })
+        )
+        if existing?.first == nil {
+            modelContext.insert(ReadStory(id: id))
+            try? modelContext.save()
+        }
     }
 
     // MARK: - Pieces