feat(podcasts): #8 follow-ups — AI summary + in-app Open Bookmark

Resolves the two deviations from the episode detail sheet:

- AI summary now shows in the detail sheet. Added a URL-keyed AISummaryStore
  that BookmarksViewModel populates as it enriches/loads bookmarks, so the
  decoupled Podcasts tab can surface a summary from just the episode URL —
  without recoupling to BookmarksViewModel.
- "Open Bookmark" opens the in-app BrowserView as a nested sheet (generator
  threaded through PodcastLibraryView / EpisodePickerView), alongside the
  existing "Open in Safari" and Share.

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

Marks/MarksApp.swift

@@ -57,7 +57,7 @@ struct MainContainer: View {
                 TagsView(viewModel: viewModel)
             }
             Tab("Podcasts", systemImage: "headphones", value: AppTab.podcasts) {
-                PodcastLibraryView(vm: viewModel.podcastPlayer, claude: viewModel.claude)
+                PodcastLibraryView(vm: viewModel.podcastPlayer, claude: viewModel.claude, podcastGenerator: viewModel.podcastGenerator)
             }
             .badge(library.unplayedCount)
             Tab(value: AppTab.search, role: .search) {