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/Views/BrowserView.swift

@@ -220,6 +220,7 @@ struct BrowserView: View {
     let initialTitle: String
     let claude: ClaudeService
     let podcastPlayer: PodcastPlayerViewModel
+    let podcastGenerator: PodcastGenerationManager
     var onArchive: (() async -> Void)? = nil
 
     @Environment(\.dismiss) private var dismiss
@@ -232,11 +233,12 @@ struct BrowserView: View {
     @State private var navForwardCount = 0
     @State private var toolbarHidden = false
 
-    init(url: URL, title: String, claude: ClaudeService, podcastPlayer: PodcastPlayerViewModel, onArchive: (() async -> Void)? = nil) {
+    init(url: URL, title: String, claude: ClaudeService, podcastPlayer: PodcastPlayerViewModel, podcastGenerator: PodcastGenerationManager, onArchive: (() async -> Void)? = nil) {
         self.initialURL = url
         self.initialTitle = title
         self.claude = claude
         self.podcastPlayer = podcastPlayer
+        self.podcastGenerator = podcastGenerator
         self.onArchive = onArchive
         self._state = State(initialValue: BrowserState(url: url))
     }
@@ -375,13 +377,23 @@ struct BrowserView: View {
             Button {
                 let currentUrl = (state.currentURL ?? initialURL).absoluteString
                 let currentTitle = state.pageTitle.isEmpty ? initialTitle : state.pageTitle
-                podcastPlayer.start(
-                    articleUrl: currentUrl,
-                    articleTitle: currentTitle,
-                    claude: claude,
-                    parentBookmarkUrl: initialURL.absoluteString
-                )
-                showPodcast = true
+                // Never interrupt current playback: play-and-show only when idle,
+                // otherwise generate in the background.
+                if podcastPlayer.currentArticleUrl.isEmpty {
+                    podcastPlayer.start(
+                        articleUrl: currentUrl,
+                        articleTitle: currentTitle,
+                        claude: claude,
+                        parentBookmarkUrl: initialURL.absoluteString
+                    )
+                    showPodcast = true
+                } else {
+                    podcastGenerator.enqueue(
+                        articleUrl: currentUrl,
+                        title: currentTitle,
+                        parentBookmarkUrl: initialURL.absoluteString
+                    )
+                }
             } label: {
                 Image(systemName: "headphones")
                     .frame(width: 44, height: 44)