Add reading progress tracking

- BrowserState: inject scroll-tracking JS after page load, report position
  via WKScriptMessageHandler (weak ref to avoid retain cycle)
- BrowserView: thin blue progress bar at top of viewport; saves progress
  to UserDefaults on dismiss; restores scroll position on revisit
- BookmarkRow: 2px progress bar below tags for partially-read articles
- BookmarksView: loads ReadingProgress.all() on appear, refreshes when
  browser sheet dismisses
- ReadingProgress: static helper for UserDefaults-backed [url: Double] store;
  removes entry when article is fully read (≥99%)

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

Marks/Views/BookmarksView.swift

@@ -11,12 +11,13 @@ struct BookmarksView: View {
     @State private var editingBookmark: Bookmark?
     @State private var browsingBookmark: Bookmark?
     @State private var podcastBookmark: Bookmark?
+    @State private var readingProgress: [String: Double] = ReadingProgress.all()
 
     var body: some View {
         NavigationStack {
             List {
                 ForEach(viewModel.bookmarks) { bookmark in
-                    BookmarkRow(bookmark: bookmark)
+                    BookmarkRow(bookmark: bookmark, readingProgress: readingProgress[bookmark.url] ?? 0)
                         .onTapGesture {
                             browsingBookmark = bookmark
                         }
@@ -158,6 +159,9 @@ struct BookmarksView: View {
                 BrowserView(url: url, title: bookmark.displayTitle, claude: viewModel.claude)
             }
         }
+        .onChange(of: browsingBookmark) { _, new in
+            if new == nil { readingProgress = ReadingProgress.all() }
+        }
         .sheet(item: $podcastBookmark) { bookmark in
             if let claude = viewModel.claude {
                 PodcastPlayerView(articleUrl: bookmark.url, claude: claude)