Add Global vendor country region

Allow vendor country selections to use a Global region alongside
existing country and EU values. The new value is accepted by backend
country-code validation, exposed through GraphQL and MCP schemas, and
shown in the shared country picker label set.

Signed-off-by: Cursor Agent <cursoragent@cursor.com>

Co-authored-by: Bryan FRIMIN <bryan@frimin.fr>

pkg/coredata/country_code.go

@@ -25,6 +25,10 @@ type (
 	CountryCode string
 )
 
+const (
+	CountryCodeGlobal CountryCode = "GLOBAL"
+)
+
 const (
 	CountryCodeAD CountryCode = "AD"
 	CountryCodeAE CountryCode = "AE"
@@ -286,6 +290,7 @@ var (
 func (v CountryCode) IsValid() bool {
 	switch v {
 	case
+		CountryCodeGlobal,
 		CountryCodeAD,
 		CountryCodeAE,
 		CountryCodeAF,