feat(antigravity): 自动设置隐私并支持后台手动重试

新增 Antigravity OAuth 隐私设置能力，在账号创建、刷新、导入和后台
Token 刷新路径自动调用 setUserSettings + fetchUserInfo 关闭遥测；
持久化后同步内存 Extra，错误处理改为日志记录。

Made-with: Cursor

backend/internal/service/antigravity_privacy_service_test.go

@@ -0,0 +1,18 @@
+//go:build unit
+
+package service
+
+import "testing"
+
+func TestApplyAntigravityPrivacyMode_SetsInMemoryExtra(t *testing.T) {
+	account := &Account{}
+
+	applyAntigravityPrivacyMode(account, AntigravityPrivacySet)
+
+	if account.Extra == nil {
+		t.Fatal("expected account.Extra to be initialized")
+	}
+	if got := account.Extra["privacy_mode"]; got != AntigravityPrivacySet {
+		t.Fatalf("expected privacy_mode %q, got %v", AntigravityPrivacySet, got)
+	}
+}