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/JarvisApp.swift

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