feat: Show annotations button only when article contains annotations

Add conditional visibility for the annotations button in the toolbar based on whether the loaded article contains any rd-annotation tags.

Changes:
- Add hasAnnotations property to BookmarkDetailViewModel
- Check for <rd-annotation tags when processing article content
- Conditionally show/hide annotations button in BookmarkDetailView2

readeck/UI/BookmarkDetail/BookmarkDetailView2.swift

@@ -270,10 +270,12 @@ struct BookmarkDetailView2: View {
                     Image(systemName: "tag")
                 }
 
-                Button(action: {
-                    showingAnnotationsSheet = true
-                }) {
-                    Image(systemName: "pencil.line")
+                if viewModel.hasAnnotations {
+                    Button(action: {
+                        showingAnnotationsSheet = true
+                    }) {
+                        Image(systemName: "pencil.line")
+                    }
                 }
 
                 Button(action: {

readeck/UI/BookmarkDetail/BookmarkDetailViewModel.swift

@@ -20,6 +20,7 @@ class BookmarkDetailViewModel {
     var settings: Settings?
     var readProgress: Int = 0
     var selectedAnnotationId: String?
+    var hasAnnotations: Bool = false
 
     private var factory: UseCaseFactory?
     private var cancellables = Set<AnyCancellable>()
@@ -85,8 +86,11 @@ class BookmarkDetailViewModel {
         let paragraphs = articleContent
             .components(separatedBy: .newlines)
             .filter { !$0.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty }
-        
+
         articleParagraphs = paragraphs
+
+        // Check if article contains annotations
+        hasAnnotations = articleContent.contains("<rd-annotation")
     }
     
     @MainActor