refactor: enhance markdown processing and code structure

- Updated the ConduitMarkdown class to streamline markdown rendering, improving maintainability and clarity.
- Refactored the markdown configuration to utilize new methods for building markdown blocks and handling LaTeX syntax.
- Improved the StreamingMarkdownWidget to leverage the updated markdown processing logic, ensuring a cohesive user experience.
- Enhanced the handling of Mermaid diagrams and LaTeX rendering, providing better support for complex markdown content.

lib/shared/widgets/markdown/code_block_header.dart

@@ -0,0 +1,49 @@
+import 'package:flutter/material.dart';
+import '../../theme/theme_extensions.dart';
+
+class CodeBlockHeader extends StatelessWidget {
+  const CodeBlockHeader({
+    super.key,
+    required this.language,
+    required this.onCopy,
+  });
+
+  final String language;
+  final VoidCallback onCopy;
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = context.conduitTheme;
+    final label = language.isEmpty ? 'code' : language;
+    return Container(
+      padding: const EdgeInsets.symmetric(
+        horizontal: Spacing.sm,
+        vertical: Spacing.xs,
+      ),
+      decoration: BoxDecoration(
+        color: theme.surfaceContainer.withValues(alpha: 0.35),
+        borderRadius: const BorderRadius.vertical(
+          top: Radius.circular(AppBorderRadius.sm),
+        ),
+      ),
+      child: Row(
+        children: [
+          Text(
+            label,
+            style: AppTypography.codeStyle.copyWith(
+              color: theme.textSecondary,
+              fontWeight: FontWeight.w600,
+            ),
+          ),
+          const Spacer(),
+          IconButton(
+            icon: const Icon(Icons.copy_rounded, size: 18),
+            color: theme.iconPrimary,
+            tooltip: 'Copy code',
+            onPressed: onCopy,
+          ),
+        ],
+      ),
+    );
+  }
+}