Replace zerolog with log/slog from stdlib

- Rewrite logger package to use slog with LevelVar for dynamic levels
- Update all packages to use *slog.Logger instead of *zerolog.Logger
- Use TextHandler for TTY (dev), JSONHandler for production
- Add make check target (runs lint + test)
- Add make test target

internal/handlers/login.go

@@ -12,7 +12,7 @@ func (s *Handlers) HandleLoginGET() http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		err := t.ExecuteTemplate(w, "login.html", nil)
 		if err != nil {
-			s.log.Error().Err(err).Msg("")
+			s.log.Error("template execution failed", "error", err)
 			http.Error(w, http.StatusText(500), 500)
 		}
 	}