feat(podcasts): surface mark-played in context menu, full player, and row badge

- Add a long-press context menu on each library row (Play/Pause,
  Mark as Played/Unplayed, Delete).
- Add a Mark as Played/Unplayed control next to Share Episode in the full
  player, tracking the loaded episode across queue advances.
- Show a green checkmark on played rows, on top of the existing Played
  section + dimmed title, so played state reads at a glance.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Marks/Services/PodcastIndex.swift

@@ -54,6 +54,10 @@ enum PodcastIndex {
         all().filter { $0.parentBookmarkUrl == bookmarkUrl || $0.articleUrl == bookmarkUrl }
     }
 
+    static func isPlayed(articleUrl: String) -> Bool {
+        all().first { $0.articleUrl == articleUrl }?.isPlayed ?? false
+    }
+
     /// Mark an episode played (default) or back to unplayed. No-op if unknown.
     static func setPlayed(articleUrl: String, _ played: Bool = true) {
         var entries = all()