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/Models/Models.swift

@@ -199,6 +199,17 @@ final class ReadStory {
     }
 }
 
+@Model
+final class SavedStory {
+    @Attribute(.unique) var id: String
+    var savedAt: Date
+
+    init(id: String) {
+        self.id = id
+        self.savedAt = Date()
+    }
+}
+
 @Model
 final class CachedArticle {
     @Attribute(.unique) var id: String