优化代码结构，修复部分bug

lib/pages/book/book_tab_page.dart

@@ -295,7 +295,7 @@ class _BookTabPageState extends State<BookTabPage> {
           style: TextStyle(fontSize: 14, color: colors.onSurface.withValues(alpha: 0.6), height: 1.5)),
       actions: [
         TextButton(onPressed: () => Navigator.pop(ctx), child: Text('取消', style: TextStyle(color: colors.onSurface.withValues(alpha: 0.6)))),
-        ElevatedButton(onPressed: () async { await context.read<AppProvider>().removeBook(book.id); Navigator.pop(ctx); _loadFirst(); },
+        ElevatedButton(onPressed: () async { await context.read<AppProvider>().removeBook(book.id); if (!ctx.mounted) return; Navigator.pop(ctx); if (mounted) _loadFirst(); },
           style: ElevatedButton.styleFrom(backgroundColor: colors.error, foregroundColor: colors.onError, elevation: 0,
               shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(8)), padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 8)),
           child: const Text('删除'),