Add swipe actions: left to save, right to share + mark read

- Swipe left bookmarks a story (toggle) into the Saved tab; swipe right opens
  the iOS share sheet and marks the story read.
- New SavedStory SwiftData model; Saved tab now lists explicit bookmarks
  instead of the whole offline cache.
- Convert the feed to a styled List (native swipeActions require it), keeping
  the flat look, pull-to-refresh, and infinite scroll. Add a ShareSheet wrapper.

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

Jarvis/Views/Shared/ShareSheet.swift

@@ -0,0 +1,15 @@
+// ShareSheet.swift
+// Jarvis — thin wrapper around UIActivityViewController for the share action.
+
+import SwiftUI
+import UIKit
+
+struct ShareSheet: UIViewControllerRepresentable {
+    let items: [Any]
+
+    func makeUIViewController(context: Context) -> UIActivityViewController {
+        UIActivityViewController(activityItems: items, applicationActivities: nil)
+    }
+
+    func updateUIViewController(_ controller: UIActivityViewController, context: Context) {}
+}