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/models/ui/ui.go

@@ -102,6 +102,11 @@ type Sections struct {
 type Footer struct {
 	ViewOnGithub string `json:"viewOnGithub"`
 	LastUpdated  string `json:"lastUpdated"`
+	Linkedin     string `json:"linkedin"`
+	Github       string `json:"github"`
+	Domestika    string `json:"domestika"`
+	Email        string `json:"email"`
+	Phone        string `json:"phone"`
 }
 
 // Portfolio labels