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/Shared/SignalStripe.swift

@@ -6,10 +6,12 @@ import SwiftUI
 struct SignalStripe: View {
     let score: Int
     var width: CGFloat = 3
+    /// Read stories drop the colored stripe for a neutral one.
+    var muted: Bool = false
 
     var body: some View {
         Rectangle()
-            .fill(Signal.stripeColor(score))
+            .fill(muted ? Palette.hairline : Signal.stripeColor(score))
             .frame(width: width)
     }
 }