fix(ui): add backend fields to CliproxyUpdateCheckResult type

Sync UI type definition with server response that now includes
backend and backendLabel fields for dynamic backend display.

ui/src/lib/api-client.ts

@@ -268,6 +268,10 @@ export interface CliproxyUpdateCheckResult {
   latestVersion: string;
   fromCache: boolean;
   checkedAt: number; // Unix timestamp of last check
+  // Backend info
+  backend: 'original' | 'plus';
+  backendLabel: string;
+  // Stability fields
   isStable: boolean; // Whether current version is at or below max stable
   maxStableVersion: string; // Maximum stable version (e.g., "6.6.80")
   stabilityMessage?: string; // Warning message if running unstable version