FRE-4763: Fix Clone() context argument and apply Code Reviewer P0-P3 fixes

- P0: Update auth header after token refresh via GetSession() + SetAuthHeader()
- P2: Unconditional req.WithContext(ctx) instead of fragile context.Background() check
- Fix: req.Clone(ctx) takes context.Context, not *http.Request (req.WithContext(ctx))
- Remove unused checkAuthenticated() and NewRequestWithContext() helpers

cmd/mail.go

@@ -15,22 +15,6 @@ import (
 	"github.com/spf13/cobra"
 )
 
-func checkAuthenticated() (*auth.Session, error) {
-	sessionMgr, err := auth.NewSessionManager()
-	if err != nil {
-		return nil, fmt.Errorf("failed to create session manager: %w", err)
-	}
-	authenticated, err := sessionMgr.IsAuthenticated()
-	if err != nil || !authenticated {
-		return nil, fmt.Errorf("not authenticated (run 'pop login' first): %w", err)
-	}
-	session, err := sessionMgr.GetSession()
-	if err != nil {
-		return nil, fmt.Errorf("not authenticated: %w", err)
-	}
-	return session, nil
-}
-
 func checkAuthenticatedWithManager() (*auth.Session, *auth.SessionManager, error) {
 	sessionMgr, err := auth.NewSessionManager()
 	if err != nil {