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/BookmarksViewModel.swift

@@ -17,6 +17,7 @@ final class BookmarksViewModel {
 
     private let api: LinkdingAPI
     let claude = ClaudeService()
+    let podcastPlayer = PodcastPlayerViewModel()
     private var enrichTask: Task<Void, Never>?
     private let cacheFileUrl: URL