Add mini-player, per-row podcast button, and resume-from-position

- MiniPlayerView: persistent bar above tab bar, shows title/progress,
  play/pause and close buttons, slides in when audio starts
- PodcastPlayerViewModel lifted to BookmarksViewModel so it persists
  across sheet dismissals — audio keeps playing in background
- Headphone button on every bookmark row (filled = cached, outline = not)
- start() is idempotent: re-tapping same article resumes without restart
- stop() saves playback position before teardown (PodcastProgress)
- setupPlayer restores saved position on next open
- BrowserView keeps its own local VM (stopOnDismiss: true)

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

Marks/Views/BookmarkRow.swift

@@ -3,6 +3,7 @@ import SwiftUI
 struct BookmarkRow: View {
     let bookmark: Bookmark
     var readingProgress: Double = 0
+    var onPodcast: (() -> Void)? = nil
 
     var body: some View {
         VStack(alignment: .leading, spacing: 6) {
@@ -29,6 +30,18 @@ struct BookmarkRow: View {
                         .font(.system(size: 16))
                         .foregroundStyle(.tertiary)
                 }
+
+                Spacer()
+
+                if let onPodcast {
+                    Button(action: onPodcast) {
+                        Image(systemName: podcastCached ? "headphones.circle.fill" : "headphones.circle")
+                            .font(.system(size: 26))
+                            .foregroundStyle(podcastCached ? .blue : Color(.systemGray3))
+                    }
+                    .buttonStyle(.plain)
+                    .padding(.top, 1)
+                }
             }
 
             if let summary = bookmark.aiSummary {
@@ -76,6 +89,10 @@ struct BookmarkRow: View {
         .contentShape(Rectangle())
     }
 
+    private var podcastCached: Bool {
+        FileManager.default.fileExists(atPath: ClaudeService.cachedPodcastURL(for: bookmark.url).path)
+    }
+
     private var effectiveTags: [String] {
         let base = bookmark.tagNames
         let ai = bookmark.aiTags ?? []