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

main.go

@@ -92,7 +92,7 @@ func run(args []string) int {
 // Returns:
 //   - true if the string appears to be a URI, false otherwise
 func isURI(str string) bool {
-	return len(str) > 7 && (str[:7] == "http://" || str[:8] == "https://")
+	return strings.HasPrefix(str, "http://") || strings.HasPrefix(str, "https://")
 }
 
 // printUsage prints the command-line usage information.