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/Views/BookmarksViewModel.swift

@@ -299,6 +299,8 @@ final class BookmarksViewModel {
             "tags": bookmark.aiTags ?? []
         ]
         UserDefaults.standard.set(store, forKey: "aiData")
+        // Mirror by URL so the decoupled Podcasts tab can show summaries.
+        AISummaryStore.set(bookmark.aiSummary, for: bookmark.url)
     }
 
     private func restoreAIData() {
@@ -311,6 +313,8 @@ final class BookmarksViewModel {
             if let d = store["\(list[i].id)"] {
                 list[i].aiSummary = (d["summary"] as? String).flatMap { $0.isEmpty ? nil : $0 }
                 list[i].aiTags = d["tags"] as? [String]
+                // Keep the URL-keyed summary cache populated for existing bookmarks.
+                if let summary = list[i].aiSummary { AISummaryStore.set(summary, for: list[i].url) }
             }
         }
     }