更新修复

call-client/src-tauri/src/commands/update.rs

@@ -543,7 +543,7 @@ pub(crate) fn check_apt_update_internal(
             log_preview("检查更新: apt-cache policy stderr(非致命，仍可能解析 stdout)", &stderr, 2000);
         }
         let mut installed_version = extract_policy_value(&stdout, "Installed:");
-        let candidate_version = extract_policy_value(&stdout, "Candidate:");
+        let mut candidate_version = extract_policy_value(&stdout, "Candidate:");
 
         if installed_version == "(unknown)" {
             let dpkg_cmd = format!("dpkg-query -W -f=${{Version}} {}", package_name);