fix(html_converter): Escape ${ in markdown for JavaScript template strings

Add proper escaping for ${ to prevent template expression interpretation
in JavaScript template literals. This fixes empty markdown content in
generated HTML files.

Author: lufftw

tools/ai-markmap-agent/src/output/html_converter.py

@@ -285,8 +285,14 @@ def convert(
         Returns:
             HTML string
         """
-        # Escape backticks and backslashes in markdown for JS template literal
-        escaped_content = markdown_content.replace("\\", "\\\\").replace("`", "\\`")
+        # Escape special characters for JavaScript template literal
+        # Order matters: escape backslash first, then backtick, then ${ to prevent template expression interpretation
+        escaped_content = (
+            markdown_content
+            .replace("\\", "\\\\")  # Escape backslashes first
+            .replace("`", "\\`")     # Escape backticks
+            .replace("${", "\\${")   # Escape ${ to prevent template expression interpretation
+        )
 
         # Prepare template variables
         template_vars = {