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

tests/integration/email_test.go

@@ -39,7 +39,7 @@ func TestSMTPConnection(t *testing.T) {
 			if err != nil {
 				t.Fatalf("TLS dial failed: %v", err)
 			}
-			defer conn.Close()
+			defer func() { _ = conn.Close() }()
 			t.Log("TLS connection established (port 465 - implicit SSL)")
 		} else {
 			// STARTTLS
@@ -47,7 +47,7 @@ func TestSMTPConnection(t *testing.T) {
 			if err != nil {
 				t.Fatalf("SMTP dial failed: %v", err)
 			}
-			defer client.Close()
+			defer func() { _ = client.Close() }()
 
 			if err := client.StartTLS(tlsConfig); err != nil {
 				t.Fatalf("STARTTLS failed: %v", err)
@@ -71,7 +71,7 @@ func TestSMTPConnection(t *testing.T) {
 			if err != nil {
 				t.Fatalf("TLS dial failed: %v", err)
 			}
-			defer conn.Close()
+			defer func() { _ = conn.Close() }()
 
 			client, err = smtp.NewClient(conn, host)
 			if err != nil {
@@ -88,7 +88,7 @@ func TestSMTPConnection(t *testing.T) {
 				t.Fatalf("STARTTLS failed: %v", err)
 			}
 		}
-		defer client.Close()
+		defer func() { _ = client.Close() }()
 
 		auth := smtp.PlainAuth("", user, pass, host)
 		if err := client.Auth(auth); err != nil {