fix(proxy): resolve 404 error and auto-setup proxy targets

Issues fixed:
1. Route prefix mismatch - Live config was set to use /claude, /codex,
   /gemini prefixes but server only had routes without prefixes
2. Proxy targets not auto-configured - start_with_takeover only modified
   Live config but didn't set is_proxy_target=true for current providers

Changes:
- Add prefixed routes (/claude/v1/messages, /codex/v1/*, /gemini/v1beta/*)
  to server.rs for backward compatibility
- Remove URL prefixes from takeover_live_configs() - server can route
  by API endpoint path alone (/v1/messages=Claude, /v1/chat/completions=Codex)
- Add setup_proxy_targets() method that automatically sets current providers
  as proxy targets when starting proxy with takeover
- Unify proxy toggle in SettingsPage to use takeover mode (same as main UI)
- Fix error message extraction in useProxyStatus hooks
- Fix provider switch logic to check both takeover flag AND proxy running state

src-tauri/src/commands/proxy.rs

@@ -15,12 +15,26 @@ pub async fn start_proxy_server(
     state.proxy_service.start().await
 }
 
+/// 启动代理服务器（带 Live 配置接管）
+#[tauri::command]
+pub async fn start_proxy_with_takeover(
+    state: tauri::State<'_, AppState>,
+) -> Result<ProxyServerInfo, String> {
+    state.proxy_service.start_with_takeover().await
+}
+
 /// 停止代理服务器
 #[tauri::command]
 pub async fn stop_proxy_server(state: tauri::State<'_, AppState>) -> Result<(), String> {
     state.proxy_service.stop().await
 }
 
+/// 停止代理服务器（恢复 Live 配置）
+#[tauri::command]
+pub async fn stop_proxy_with_restore(state: tauri::State<'_, AppState>) -> Result<(), String> {
+    state.proxy_service.stop_with_restore().await
+}
+
 /// 获取代理服务器状态
 #[tauri::command]
 pub async fn get_proxy_status(state: tauri::State<'_, AppState>) -> Result<ProxyStatus, String> {
@@ -48,6 +62,25 @@ pub async fn is_proxy_running(state: tauri::State<'_, AppState>) -> Result<bool,
     Ok(state.proxy_service.is_running().await)
 }
 
+/// 检查是否处于 Live 接管模式
+#[tauri::command]
+pub async fn is_live_takeover_active(state: tauri::State<'_, AppState>) -> Result<bool, String> {
+    state.proxy_service.is_takeover_active().await
+}
+
+/// 代理模式下切换供应商（热切换）
+#[tauri::command]
+pub async fn switch_proxy_provider(
+    state: tauri::State<'_, AppState>,
+    app_type: String,
+    provider_id: String,
+) -> Result<(), String> {
+    state
+        .proxy_service
+        .switch_proxy_target(&app_type, &provider_id)
+        .await
+}
+
 // ==================== 故障转移相关命令 ====================
 
 /// 获取代理目标列表