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/BookmarkRow.swift

@@ -2,6 +2,7 @@ import SwiftUI
 
 struct BookmarkRow: View {
     let bookmark: Bookmark
+    var readingProgress: Double = 0
 
     var body: some View {
         VStack(alignment: .leading, spacing: 6) {
@@ -54,6 +55,22 @@ struct BookmarkRow: View {
                 }
                 .padding(.leading, 40)
             }
+
+            // Reading progress indicator
+            if readingProgress > 0.02 {
+                GeometryReader { geo in
+                    ZStack(alignment: .leading) {
+                        Rectangle()
+                            .fill(Color(.systemGray5))
+                        Rectangle()
+                            .fill(Color.blue.opacity(0.55))
+                            .frame(width: geo.size.width * min(readingProgress, 1))
+                    }
+                }
+                .frame(height: 2)
+                .clipShape(Capsule())
+                .padding(.leading, 40)
+            }
         }
         .padding(.vertical, 14)
         .contentShape(Rectangle())