Fix Books navigation — tapping a book no longer re-pushes the library

BookLibraryView is itself pushed from PracticeView's NavigationStack,
so the .navigationDestination(for: Book.self) it declared was a
non-root registration. Combined with NavigationLink(value: book), that
resolved the push to *both* the destination handler and the closure
that produced BookLibraryView originally — pushing the chapter list
underneath, then re-pushing the library on top. Hitting back popped
the library and revealed the chapter list, in the wrong order.

Switched both Library→ChapterList and ChapterList→Reader to closure-
based NavigationLinks. Destinations attach directly to the link, no
type-keyed registry involved.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Conjuga/Conjuga/Views/Practice/Books/BookChapterListView.swift

@@ -19,7 +19,9 @@ struct BookChapterListView: View {
     var body: some View {
         List {
             ForEach(allChapters) { chapter in
-                NavigationLink(value: chapter) {
+                NavigationLink {
+                    BookReaderView(chapter: chapter)
+                } label: {
                     HStack(spacing: 12) {
                         Text("\(chapter.number)")
                             .font(.subheadline.weight(.bold).monospacedDigit())

Conjuga/Conjuga/Views/Practice/Books/BookLibraryView.swift

@@ -17,7 +17,9 @@ struct BookLibraryView: View {
                 ScrollView {
                     LazyVStack(spacing: 12) {
                         ForEach(books) { book in
-                            NavigationLink(value: book) {
+                            NavigationLink {
+                                BookChapterListView(book: book)
+                            } label: {
                                 BookCard(book: book)
                             }
                             .tint(.primary)
@@ -29,12 +31,6 @@ struct BookLibraryView: View {
         }
         .navigationTitle("Books")
         .navigationBarTitleDisplayMode(.inline)
-        .navigationDestination(for: Book.self) { book in
-            BookChapterListView(book: book)
-        }
-        .navigationDestination(for: BookChapter.self) { chapter in
-            BookReaderView(chapter: chapter)
-        }
     }
 }