fix: Use total content height for read progress calculation

Added ContentHeightPreferenceKey to track the total ScrollView content height.

The bug: Progress was calculated using only webViewHeight - containerHeight,
which ignores the header, title, and other content above the webview.

The fix: Use total content height (header + title + webview + archive section)
instead of just webViewHeight for accurate progress calculation.

Changes:
- Added ContentHeightPreferenceKey preference key
- Added contentHeight state variable
- Added background GeometryReader to VStack to measure total content height
- Changed progress calculation: contentHeight - containerHeight (not webViewHeight)

Applied to both BookmarkDetailLegacyView and BookmarkDetailView2.

readeck/UI/BookmarkDetail/BookmarkDetailLegacyView.swift

@@ -9,6 +9,14 @@ struct ScrollOffsetPreferenceKey: PreferenceKey {
     }
 }
 
+// PreferenceKey for content height tracking
+struct ContentHeightPreferenceKey: PreferenceKey {
+    static var defaultValue: CGFloat = 0
+    static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) {
+        value = nextValue()
+    }
+}
+
 struct BookmarkDetailLegacyView: View {
     let bookmarkId: String
     @Binding var useNativeWebView: Bool
@@ -17,6 +25,7 @@ struct BookmarkDetailLegacyView: View {
 
     @State private var viewModel: BookmarkDetailViewModel
     @State private var webViewHeight: CGFloat = 300
+    @State private var contentHeight: CGFloat = 0
     @State private var showingFontSettings = false
     @State private var showingLabelsSheet = false
     @State private var readingProgress: Double = 0.0
@@ -113,16 +122,27 @@ struct BookmarkDetailLegacyView: View {
                         .frame(maxWidth: .infinity)
                     }
                     }
+                    .background(
+                        GeometryReader { contentGeo in
+                            Color.clear.preference(
+                                key: ContentHeightPreferenceKey.self,
+                                value: contentGeo.size.height
+                            )
+                        }
+                    )
                 }
                 .coordinateSpace(name: "scrollView")
                 .clipped()
                 .ignoresSafeArea(edges: .top)
                 .scrollPosition($scrollPosition)
+                .onPreferenceChange(ContentHeightPreferenceKey.self) { height in
+                    contentHeight = height
+                }
                 .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 = webViewHeight - containerHeight
+                    let maxOffset = contentHeight - containerHeight
 
                     guard maxOffset > 0 else { return }
 

readeck/UI/BookmarkDetail/BookmarkDetailView2.swift

@@ -10,6 +10,7 @@ struct BookmarkDetailView2: View {
 
     @State private var viewModel: BookmarkDetailViewModel
     @State private var webViewHeight: CGFloat = 300
+    @State private var contentHeight: CGFloat = 0
     @State private var showingFontSettings = false
     @State private var showingLabelsSheet = false
     @State private var readingProgress: Double = 0.0
@@ -120,16 +121,27 @@ struct BookmarkDetailView2: View {
                         .frame(maxWidth: .infinity)
                     }
                 }
+                .background(
+                    GeometryReader { contentGeo in
+                        Color.clear.preference(
+                            key: ContentHeightPreferenceKey.self,
+                            value: contentGeo.size.height
+                        )
+                    }
+                )
             }
             .coordinateSpace(name: "scrollView")
             .clipped()
             .ignoresSafeArea(edges: .top)
             .scrollPosition($scrollPosition)
+            .onPreferenceChange(ContentHeightPreferenceKey.self) { height in
+                contentHeight = height
+            }
             .onPreferenceChange(ScrollOffsetPreferenceKey.self) { offset in
                 // Calculate progress from scroll offset
                 let scrollOffset = -offset.y
                 let containerHeight = geometry.size.height
-                let maxOffset = webViewHeight - containerHeight
+                let maxOffset = contentHeight - containerHeight
 
                 guard maxOffset > 0 else { return }