feat (auth): CLI OAuth supports pasting callback URLs to complete login

- Added callback URL resolution and terminal prompt logic
  - Codex/Claude/iFlow/Antigravity/Gemini login supports callback URL or local callback completion
  - Update Gemini login option signature and manager call
  - CLI default prompt function is compatible with null input to continue waiting

sdk/auth/iflow.go

@@ -84,9 +84,32 @@ func (a *IFlowAuthenticator) Login(ctx context.Context, cfg *config.Config, opts
 
 	fmt.Println("Waiting for iFlow authentication callback...")
 
-	result, err := oauthServer.WaitForCallback(5 * time.Minute)
-	if err != nil {
+	callbackCh := make(chan *iflow.OAuthResult, 1)
+	callbackErrCh := make(chan error, 1)
+	manualCh, manualErrCh := promptForOAuthCallback(opts.Prompt, "iFlow")
+
+	go func() {
+		result, errWait := oauthServer.WaitForCallback(5 * time.Minute)
+		if errWait != nil {
+			callbackErrCh <- errWait
+			return
+		}
+		callbackCh <- result
+	}()
+
+	var result *iflow.OAuthResult
+	select {
+	case result = <-callbackCh:
+	case err = <-callbackErrCh:
 		return nil, fmt.Errorf("iflow auth: callback wait failed: %w", err)
+	case manual := <-manualCh:
+		result = &iflow.OAuthResult{
+			Code:  manual.Code,
+			State: manual.State,
+			Error: manual.Error,
+		}
+	case err = <-manualErrCh:
+		return nil, err
 	}
 	if result.Error != "" {
 		return nil, fmt.Errorf("iflow auth: provider returned error %s", result.Error)