fix: resolve golangci-lint errcheck and staticcheck warnings

Use t.Setenv in tests, add error return handling, and replace
WriteString(fmt.Sprintf(...)) with fmt.Fprintf.

internal/handlers/cv_pages_test.go

@@ -128,7 +128,7 @@ func TestDefaultCVShortcut(t *testing.T) {
 	if err != nil || resp.StatusCode != http.StatusOK {
 		t.Skip("Skipping PDF generation test - server not running on localhost:1999")
 	}
-	resp.Body.Close()
+	_ = resp.Body.Close()
 
 	handler := newTestCVHandler(t, "localhost:1999", nil)