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/MarksApp.swift

@@ -65,18 +65,19 @@ struct MainContainer: View {
         .onOpenURL { url in
             handleDeepLink(url)
         }
-        .task { applyPendingIntent() }
+        .task { applyPendingIntent(); viewModel.processPendingPodcastRequests() }
         .onChange(of: router.openBookmarkURL) { _, _ in applyPendingIntent() }
         .onChange(of: router.searchRequest) { _, _ in applyPendingIntent() }
         .onChange(of: scenePhase) { old, new in
             // Refresh when returning to the foreground (cold launch is already
             // covered by BookmarksView's .task, so only react to a real re-entry).
             if new == .active && old != .active {
+                viewModel.processPendingPodcastRequests()
                 Task { await viewModel.load() }
             }
         }
         .sheet(item: $deepLinkBrowser) { item in
-            BrowserView(url: item.url, title: item.url.host ?? "", claude: viewModel.claude, podcastPlayer: viewModel.podcastPlayer)
+            BrowserView(url: item.url, title: item.url.host ?? "", claude: viewModel.claude, podcastPlayer: viewModel.podcastPlayer, podcastGenerator: viewModel.podcastGenerator)
         }
         .sheet(isPresented: $showDeepLinkPlayer) {
             PodcastPlayerView(