Migrate AI features to backend, add podcast generation

- ClaudeService: replace direct OpenRouter calls with backend endpoints
  (/v1/marks/enrich, /v1/marks/search, /v1/marks/collections)
- ClaudeService: add podcast methods (generate, status, downloadAudio)
- SettingsView: replace OpenRouter key field with backend URL + API key
- PodcastPlayerView: new sheet — generate → poll → AVPlayer playback
- BrowserView: headphones toolbar button triggers podcast for current URL
- BookmarksView: "Convert to Podcast" context menu item + sheet

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Marks/Views/BookmarksView.swift

@@ -10,6 +10,7 @@ struct BookmarksView: View {
     @State private var showAddBookmark = false
     @State private var editingBookmark: Bookmark?
     @State private var browsingBookmark: Bookmark?
+    @State private var podcastBookmark: Bookmark?
 
     var body: some View {
         NavigationStack {
@@ -59,6 +60,13 @@ struct BookmarksView: View {
                             } label: {
                                 Label("Open in Safari", systemImage: "safari")
                             }
+                            if viewModel.claude != nil {
+                                Button {
+                                    podcastBookmark = bookmark
+                                } label: {
+                                    Label("Convert to Podcast", systemImage: "headphones")
+                                }
+                            }
                             Divider()
                             Button {
                                 Task { await viewModel.archive(bookmark) }
@@ -147,7 +155,12 @@ struct BookmarksView: View {
         }
         .sheet(item: $browsingBookmark) { bookmark in
             if let url = URL(string: bookmark.url) {
-                BrowserView(url: url, title: bookmark.displayTitle)
+                BrowserView(url: url, title: bookmark.displayTitle, claude: viewModel.claude)
+            }
+        }
+        .sheet(item: $podcastBookmark) { bookmark in
+            if let claude = viewModel.claude {
+                PodcastPlayerView(articleUrl: bookmark.url, claude: claude)
             }
         }
         .refreshable {