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

data/ui-en.json

@@ -36,7 +36,12 @@
   },
   "footer": {
     "viewOnGithub": "View this project on GitHub",
-    "lastUpdated": "Last updated"
+    "lastUpdated": "Last updated",
+    "linkedin": "linkedin_",
+    "github": "github_",
+    "domestika": "domestika_",
+    "email": "email@",
+    "phone": "phone#"
   },
   "portfolio": {
     "seeAllProjects": "See all projects on my",