feat(podcasts): played/unplayed queue, background generation, share-sheet audio

Turns the Podcasts tab into a proper podcast-app experience across three
features that compose on a shared background-generation service.

Background generation (#2)
- New PodcastGenerationManager runs generate→poll→download off the player,
  so producing a new episode never stops current playback.
- Headphone taps never interrupt: idle → generate-and-play in the foreground
  (unchanged); already playing → generate in the background, episode lands in
  the library. A "Generating…" banner surfaces active jobs.

Played/unplayed + queue (#1)
- PodcastEntry gains playedAt (old index.json decodes as unplayed).
- Podcasts tab splits into Up Next / Played with a Play All queue that
  auto-advances; finishing marks played; swipe to toggle played state.

Share-extension audio (#3)
- "Create podcast" toggle on the save card, plus configurable Auto-Podcast
  Tags in Settings. The extension enqueues a request into the App Group; the
  main app drains it on launch/foreground and generates in the background.

Closes #1
Closes #2
Closes #3

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

Marks/Views/BookmarksViewModel.swift

@@ -18,6 +18,7 @@ final class BookmarksViewModel {
     private let api: LinkdingAPI
     let claude = ClaudeService()
     let podcastPlayer = PodcastPlayerViewModel()
+    let podcastGenerator = PodcastGenerationManager()
     private var enrichTask: Task<Void, Never>?
     private let cacheFileUrl: URL
 
@@ -29,6 +30,32 @@ final class BookmarksViewModel {
         self.cacheFileUrl = dir.appendingPathComponent("\(name).json")
     }
 
+    /// True while the player is showing an episode (playing or generating one).
+    var isPlayerBusy: Bool { !podcastPlayer.currentArticleUrl.isEmpty }
+
+    /// Handle a headphone tap without ever interrupting current playback.
+    /// If the player is idle, generate-and-play in the foreground and return
+    /// `true` (caller should present the full player). If the player is busy,
+    /// generate in the background — the finished episode lands in the library —
+    /// and return `false`.
+    @discardableResult
+    func playOrGeneratePodcast(articleUrl: String, title: String, parentBookmarkUrl: String? = nil) -> Bool {
+        if isPlayerBusy {
+            podcastGenerator.enqueue(articleUrl: articleUrl, title: title, parentBookmarkUrl: parentBookmarkUrl)
+            return false
+        }
+        podcastPlayer.start(articleUrl: articleUrl, articleTitle: title, claude: claude, parentBookmarkUrl: parentBookmarkUrl)
+        return true
+    }
+
+    /// Drain podcast requests queued by the share extension and start generating
+    /// them in the background. Safe to call repeatedly (the queue is cleared).
+    func processPendingPodcastRequests() {
+        for req in PodcastRequests.drain() {
+            podcastGenerator.enqueue(articleUrl: req.url, title: req.title)
+        }
+    }
+
     func load(useCache: Bool = true) async {
         if useCache { loadFromCache() }
         isLoading = true