feat: Implement performant scroll progress tracking with PreferenceKey

Replace onScrollGeometryChange/onScrollPhaseChange with ContentHeightPreferenceKey
approach for improved scroll performance and accurate read progress tracking.

Changes:
- Add ScrollOffsetPreferenceKey and ContentHeightPreferenceKey for scroll tracking
- Track content end position dynamically as WebView loads
- Calculate progress from scroll offset relative to total scrollable content
- Implement 3% threshold for progress updates to reduce API calls
- Add progress locking at 100% to prevent pixel-variation regressions
- Guarantee 100% update when reaching end of content
- Apply to both BookmarkDetailLegacyView and BookmarkDetailView2

Technical approach:
- Place invisible marker at end of content to measure position in scrollView
- Update initialContentEndPosition as content grows during WebView loading
- Progress = (initialPosition - currentPosition) / (initialPosition - containerHeight)
- Lock progress once 100% reached to avoid 100% -> 99% fluctuations

readeck/UI/BookmarkDetail/BookmarkDetailLegacyView.swift

@@ -25,7 +25,8 @@ struct BookmarkDetailLegacyView: View {
 
     @State private var viewModel: BookmarkDetailViewModel
     @State private var webViewHeight: CGFloat = 300
-    @State private var contentHeight: CGFloat = 0
+    @State private var contentEndPosition: CGFloat = 0
+    @State private var initialContentEndPosition: CGFloat = 0
     @State private var showingFontSettings = false
     @State private var showingLabelsSheet = false
     @State private var readingProgress: Double = 0.0
@@ -121,42 +122,76 @@ struct BookmarkDetailLegacyView: View {
                         }
                         .frame(maxWidth: .infinity)
                     }
-                    }
+
+                        // Invisible marker to measure total content height - placed AFTER all content
+                        Color.clear
+                            .frame(height: 1)
                             .background(
-                        GeometryReader { contentGeo in
+                                GeometryReader { endGeo in
                                     Color.clear.preference(
                                         key: ContentHeightPreferenceKey.self,
-                                value: contentGeo.size.height
+                                        value: endGeo.frame(in: .named("scrollView")).maxY
                                     )
                                 }
                             )
                     }
+                }
                 .coordinateSpace(name: "scrollView")
                 .clipped()
                 .ignoresSafeArea(edges: .top)
                 .scrollPosition($scrollPosition)
-                .onPreferenceChange(ContentHeightPreferenceKey.self) { height in
+                .onPreferenceChange(ContentHeightPreferenceKey.self) { endPosition in
-                    contentHeight = height
+                    contentEndPosition = endPosition
-                }
+
-                .onPreferenceChange(ScrollOffsetPreferenceKey.self) { offset in
-                    // Calculate progress from scroll offset
-                    let scrollOffset = -offset.y  // Negative because scroll goes down
                     let containerHeight = geometry.size.height
-                    let maxOffset = contentHeight - containerHeight
 
-                    guard maxOffset > 0 else { return }
+                    // Update initial position if content grows (WebView still loading) or first time
+                    // We always take the maximum position seen (when scrolled to top, this is total content height)
+                    if endPosition > initialContentEndPosition && endPosition > containerHeight * 1.2 {
+                        initialContentEndPosition = endPosition
+                        print("📏 Content end position updated: \(Int(endPosition)) (container: \(Int(containerHeight)))")
+                    }
 
-                    let rawProgress = scrollOffset / maxOffset
+                    // Calculate progress from how much the end marker has moved up
-                    let progress = min(max(rawProgress, 0), 1)
+                    guard initialContentEndPosition > 0 else {
+                        print("⏳ Waiting for content to load... current: \(Int(endPosition)), container: \(Int(containerHeight))")
+                        return
+                    }
 
-                    // Only update if change >= 3%
+                    let totalScrollableDistance = initialContentEndPosition - containerHeight
+
+                    guard totalScrollableDistance > 0 else {
+                        print("⚠️ Content not scrollable: initial=\(initialContentEndPosition), container=\(containerHeight)")
+                        return
+                    }
+
+                    // How far has the marker moved from its initial position?
+                    let scrolled = initialContentEndPosition - endPosition
+                    let rawProgress = scrolled / totalScrollableDistance
+                    var progress = min(max(rawProgress, 0), 1)
+
+                    // Lock progress at 100% once reached (don't go back to 99% due to pixel variations)
+                    if lastSentProgress >= 0.995 {
+                        progress = max(progress, 1.0)
+                    }
+
+                    print("📊 Progress: \(Int(progress * 100))% | scrolled: \(Int(scrolled)) / \(Int(totalScrollableDistance)) | endPos: \(Int(endPosition))")
+
+                    // Check if we should update: threshold OR reaching 100% for first time
                     let threshold: Double = 0.03
-                    if abs(progress - lastSentProgress) >= threshold {
+                    let reachedEnd = progress >= 1.0 && lastSentProgress < 1.0
+                    let shouldUpdate = abs(progress - lastSentProgress) >= threshold || reachedEnd
+
+                    if shouldUpdate {
+                        print("✅ Updating progress: \(Int(lastSentProgress * 100))% → \(Int(progress * 100))%\(reachedEnd ? " [END]" : "")")
                         lastSentProgress = progress
                         readingProgress = progress
                         viewModel.debouncedUpdateReadProgress(id: bookmarkId, progress: progress, anchor: nil)
                     }
                 }
+                .onPreferenceChange(ScrollOffsetPreferenceKey.self) { _ in
+                    // Not needed anymore, we track via ContentHeightPreferenceKey
+                }
             }
         }
         .frame(maxWidth: .infinity)

readeck/UI/BookmarkDetail/BookmarkDetailView2.swift

@@ -10,7 +10,8 @@ struct BookmarkDetailView2: View {
 
     @State private var viewModel: BookmarkDetailViewModel
     @State private var webViewHeight: CGFloat = 300
-    @State private var contentHeight: CGFloat = 0
+    @State private var contentEndPosition: CGFloat = 0
+    @State private var initialContentEndPosition: CGFloat = 0
     @State private var showingFontSettings = false
     @State private var showingLabelsSheet = false
     @State private var readingProgress: Double = 0.0
@@ -120,42 +121,66 @@ struct BookmarkDetailView2: View {
                         }
                         .frame(maxWidth: .infinity)
                     }
-                }
+
+                    // Invisible marker to measure total content height - placed AFTER all content
+                    Color.clear
+                        .frame(height: 1)
                         .background(
-                    GeometryReader { contentGeo in
+                            GeometryReader { endGeo in
                                 Color.clear.preference(
                                     key: ContentHeightPreferenceKey.self,
-                            value: contentGeo.size.height
+                                    value: endGeo.frame(in: .named("scrollView")).maxY
                                 )
                             }
                         )
                 }
+            }
             .coordinateSpace(name: "scrollView")
             .clipped()
             .ignoresSafeArea(edges: .top)
             .scrollPosition($scrollPosition)
-            .onPreferenceChange(ContentHeightPreferenceKey.self) { height in
+            .onPreferenceChange(ContentHeightPreferenceKey.self) { endPosition in
-                contentHeight = height
+                contentEndPosition = endPosition
-            }
+
-            .onPreferenceChange(ScrollOffsetPreferenceKey.self) { offset in
-                // Calculate progress from scroll offset
-                let scrollOffset = -offset.y
                 let containerHeight = geometry.size.height
-                let maxOffset = contentHeight - containerHeight
 
-                guard maxOffset > 0 else { return }
+                // Update initial position if content grows (WebView still loading) or first time
+                // We always take the maximum position seen (when scrolled to top, this is total content height)
+                if endPosition > initialContentEndPosition && endPosition > containerHeight * 1.2 {
+                    initialContentEndPosition = endPosition
+                }
 
-                let rawProgress = scrollOffset / maxOffset
+                // Calculate progress from how much the end marker has moved up
-                let progress = min(max(rawProgress, 0), 1)
+                guard initialContentEndPosition > 0 else { return }
 
-                // Only update if change >= 3%
+                let totalScrollableDistance = initialContentEndPosition - containerHeight
+
+                guard totalScrollableDistance > 0 else { return }
+
+                // How far has the marker moved from its initial position?
+                let scrolled = initialContentEndPosition - endPosition
+                let rawProgress = scrolled / totalScrollableDistance
+                var progress = min(max(rawProgress, 0), 1)
+
+                // Lock progress at 100% once reached (don't go back to 99% due to pixel variations)
+                if lastSentProgress >= 0.995 {
+                    progress = max(progress, 1.0)
+                }
+
+                // Check if we should update: threshold OR reaching 100% for first time
                 let threshold: Double = 0.03
-                if abs(progress - lastSentProgress) >= threshold {
+                let reachedEnd = progress >= 1.0 && lastSentProgress < 1.0
+                let shouldUpdate = abs(progress - lastSentProgress) >= threshold || reachedEnd
+
+                if shouldUpdate {
                     lastSentProgress = progress
                     readingProgress = progress
                     viewModel.debouncedUpdateReadProgress(id: bookmarkId, progress: progress, anchor: nil)
                 }
             }
+            .onPreferenceChange(ScrollOffsetPreferenceKey.self) { _ in
+                // Not needed anymore, we track via ContentHeightPreferenceKey
+            }
         }
     }