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/BrowserView.swift

@@ -115,15 +115,18 @@ private struct WebViewRepresentable: UIViewRepresentable {
 struct BrowserView: View {
     let initialURL: URL
     let initialTitle: String
+    let claude: ClaudeService?
 
     @Environment(\.dismiss) private var dismiss
     @Environment(\.openURL) private var openURL
     @State private var state: BrowserState
     @State private var readerMode = false
+    @State private var showPodcast = false
 
-    init(url: URL, title: String) {
+    init(url: URL, title: String, claude: ClaudeService? = nil) {
         self.initialURL = url
         self.initialTitle = title
+        self.claude = claude
         self._state = State(initialValue: BrowserState(url: url))
     }
 
@@ -187,11 +190,28 @@ struct BrowserView: View {
 
                     Spacer()
 
+                    if claude != nil {
+                        Button {
+                            showPodcast = true
+                        } label: {
+                            Image(systemName: "headphones")
+                        }
+                        .disabled(state.isLoading)
+
+                        Spacer()
+                    }
+
                     ShareLink(item: state.currentURL ?? initialURL) {
                         Image(systemName: "square.and.arrow.up")
                     }
                 }
             }
         }
+        .sheet(isPresented: $showPodcast) {
+            if let claude {
+                let url = (state.currentURL ?? initialURL).absoluteString
+                PodcastPlayerView(articleUrl: url, claude: claude)
+            }
+        }
     }
 }