fix: update antigravity user-agent version to 1.19.6

Update the default user-agent version from 1.18.4 to 1.19.6
to match the latest official antigravity client.

backend/internal/pkg/antigravity/oauth.go

@@ -49,8 +49,8 @@ const (
 	antigravityDailyBaseURL = "https://daily-cloudcode-pa.sandbox.googleapis.com"
 )
 
-// defaultUserAgentVersion 可通过环境变量 ANTIGRAVITY_USER_AGENT_VERSION 配置，默认 1.18.4
-var defaultUserAgentVersion = "1.18.4"
+// defaultUserAgentVersion 可通过环境变量 ANTIGRAVITY_USER_AGENT_VERSION 配置，默认 1.19.6
+var defaultUserAgentVersion = "1.19.6"
 
 // defaultClientSecret 可通过环境变量 ANTIGRAVITY_OAUTH_CLIENT_SECRET 配置
 var defaultClientSecret = "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"

backend/internal/pkg/antigravity/oauth_test.go

@@ -690,7 +690,7 @@ func TestConstants_值正确(t *testing.T) {
 	if RedirectURI != "http://localhost:8085/callback" {
 		t.Errorf("RedirectURI 不匹配: got %s", RedirectURI)
 	}
-	if GetUserAgent() != "antigravity/1.18.4 windows/amd64" {
+	if GetUserAgent() != "antigravity/1.19.6 windows/amd64" {
 		t.Errorf("UserAgent 不匹配: got %s", GetUserAgent())
 	}
 	if SessionTTL != 30*time.Minute {