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/Services/AISummaryStore.swift

@@ -0,0 +1,27 @@
+import Foundation
+
+/// URL-keyed cache of AI summaries so views that only know a URL — e.g. the
+/// deliberately-decoupled Podcasts tab — can show a bookmark's summary without
+/// holding the full `Bookmark`. Written by `BookmarksViewModel` as it enriches
+/// and loads bookmarks; read by the episode detail sheet.
+enum AISummaryStore {
+    private static let key = "aiSummaryByURL"
+
+    static func summary(for url: String) -> String? {
+        let store = UserDefaults.standard.dictionary(forKey: key) as? [String: String] ?? [:]
+        let value = store[url]
+        return (value?.isEmpty ?? true) ? nil : value
+    }
+
+    static func set(_ summary: String?, for url: String) {
+        var store = UserDefaults.standard.dictionary(forKey: key) as? [String: String] ?? [:]
+        if let summary, !summary.isEmpty {
+            guard store[url] != summary else { return }   // no-op if unchanged
+            store[url] = summary
+        } else {
+            guard store[url] != nil else { return }
+            store.removeValue(forKey: url)
+        }
+        UserDefaults.standard.set(store, forKey: key)
+    }
+}