feat(podcasts): add a Podcasts tab to list and play on demand

Surface every podcast in PodcastIndex as a top-level tab instead of a
buried sheet. Repurpose PodcastLibraryView into the tab: play/pause toggle
per episode, a now-playing bar that opens the full player, swipe/Edit to
delete. Uses the shared podcastPlayer so playback state is consistent
across tabs. Removes the redundant Podcasts entry from the bookmarks menu.

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

Marks/MarksApp.swift

@@ -55,6 +55,9 @@ struct MainContainer: View {
             Tab("Tags", systemImage: "tag", value: AppTab.tags) {
                 TagsView(viewModel: viewModel)
             }
+            Tab("Podcasts", systemImage: "headphones", value: AppTab.podcasts) {
+                PodcastLibraryView(vm: viewModel.podcastPlayer, claude: viewModel.claude)
+            }
             Tab(value: AppTab.search, role: .search) {
                 SearchView(viewModel: viewModel)
             }