feat: add Udemy courses and fix footer i18n + golangci-lint errors

- Add 5 Udemy courses with PDF certificate links (Go, Fyne, HTMX)
- Fix cv-footer.html to use CV data instead of hardcoded values
- Add i18n labels for footer (linkedin, github, domestika, email, phone)
- Fix 11 golangci-lint errors:
  - errcheck: proper Close() error handling in email/security/tests
  - staticcheck: simplify return and merge variable declaration
  - unused: remove legacy sendEmail and formatMessage functions

internal/middleware/security_logger.go

@@ -132,7 +132,11 @@ func logToSecurityFile(eventJSON []byte) {
 		log.Printf("WARNING: Failed to open security log file: %v", err)
 		return
 	}
-	defer f.Close()
+	defer func() {
+		if err := f.Close(); err != nil {
+			log.Printf("WARNING: Failed to close security log file: %v", err)
+		}
+	}()
 
 	// Write event with newline
 	if _, err := f.Write(eventJSON); err != nil {