chore: Improve code quality and address linter feedback

This commit introduces several improvements across the codebase, primarily focused on enhancing performance, robustness, and developer experience based on static analysis feedback.

- Replaces `WriteString(fmt.Sprintf())` with the more performant `fmt.Fprintf` in the HTML and Markdown exporters.
- Enhances deferred `Close()` operations to log warnings on failure instead of silently ignoring potential I/O issues.
- Explicitly discards non-critical errors in test suites, particularly during file cleanup, to satisfy linters and clarify intent.
- Suppresses command echoing in `Taskfile.yml` for cleaner output during development tasks.

internal/exporters/docx.go

@@ -76,7 +76,15 @@ func (e *DocxExporter) Export(course *models.Course, outputPath string) error {
 	if err != nil {
 		return fmt.Errorf("failed to create output file: %w", err)
 	}
-	defer file.Close()
+	// Ensure file is closed even if WriteTo fails. Close errors are logged but not
+	// fatal since the document content has already been written to disk. A close
+	// error typically indicates a filesystem synchronization issue that doesn't
+	// affect the validity of the exported file.
+	defer func() {
+		if err := file.Close(); err != nil {
+			fmt.Fprintf(os.Stderr, "warning: failed to close output file: %v\n", err)
+		}
+	}()
 
 	// Save the document
 	_, err = doc.WriteTo(file)