refactor(proxy): simplify provider selection to use is_current directly

Changes:
- Modify provider_router to select provider based on is_current flag
  instead of is_proxy_target queue
- Remove proxy target toggle UI from ProviderCard
- Remove proxyPriority and allProviders props from ProviderList
- Remove isProxyTarget prop from ProviderHealthBadge
- Use start_with_takeover() for auto-start to ensure proper setup

This simplifies the proxy architecture by directly using the current
provider for proxying, eliminating the need for separate proxy target
management. Switching providers now immediately takes effect in proxy
mode.

src/components/proxy/ProxyPanel.tsx

@@ -302,10 +302,9 @@ function ProviderQueueItem({
           </span>
         )}
       </div>
-      {/* 健康徽章：队列中的代理目标始终显示，没有健康数据时默认为正常 */}
+      {/* 健康徽章 */}
       <ProviderHealthBadge
         consecutiveFailures={health?.consecutive_failures ?? 0}
-        isProxyTarget={true}
       />
     </div>
   );