fix: 修复 Gemini 授权链接生成失败并改进错误提示

backend/internal/handler/admin/gemini_oauth_handler.go

@@ -61,7 +61,11 @@ func (h *GeminiOAuthHandler) GenerateAuthURL(c *gin.Context) {
 	if err != nil {
 		msg := err.Error()
 		// Treat missing/invalid OAuth client configuration as a user/config error.
-		if strings.Contains(msg, "OAuth client not configured") || strings.Contains(msg, "requires your own OAuth Client") {
+		if strings.Contains(msg, "OAuth client not configured") ||
+			strings.Contains(msg, "requires your own OAuth Client") ||
+			strings.Contains(msg, "requires a custom OAuth Client") ||
+			strings.Contains(msg, "GEMINI_CLI_OAUTH_CLIENT_SECRET_MISSING") ||
+			strings.Contains(msg, "built-in Gemini CLI OAuth client_secret is not configured") {
 			response.BadRequest(c, "Failed to generate auth URL: "+msg)
 			return
 		}