refactor: improve code quality and consistency

- parser.go: compile regex once at package level (perf)
- parser.go: include response body in HTTP error messages (debug)
- main.go: use strings.HasPrefix for URI detection (safety)
- html.go: handle file close errors consistently
- docx.go: extract font size magic numbers to constants
- markdown.go: normalize item types to lowercase for consistency

internal/exporters/output.md

@@ -4,8 +4,9 @@ Course description
 
 ## Course Information
 
-- **Course ID**:
+- **Course ID**: 
 - **Share ID**: example-id
-- **Navigation Mode**:
+- **Navigation Mode**: 
 
 ---
+