fix(account): seed a fresh guest's interface language from the client locale

A guest was provisioned with only its detected time zone; its PreferredLanguage
fell to the 'en' column default. The client already sends its locale in the guest
login (GuestLoginRequest.locale), but the gateway dropped it and ProvisionGuest
took no language — so a Russian user's brand-new native guest got English
language-dependent server content (the ad banner, bot messages) until the client's
later language reconcile. Read the locale in the gateway guest handler, thread it
through GuestAuth -> ProvisionGuest, and seed PreferredLanguage from it (validated;
an unsupported/absent value keeps the 'en' default). Wire field already present —
no schema change.

backend/internal/inttest/helpers.go

@@ -154,7 +154,7 @@ func provisionAccount(t *testing.T) uuid.UUID {
 // provisionGuest creates a fresh ephemeral guest account and returns its id.
 func provisionGuest(t *testing.T) uuid.UUID {
 	t.Helper()
-	acc, err := account.NewStore(testDB).ProvisionGuest(context.Background(), "")
+	acc, err := account.NewStore(testDB).ProvisionGuest(context.Background(), "", "")
 	if err != nil {
 		t.Fatalf("provision guest: %v", err)
 	}