fix(proxy): resolve actual port for ephemeral (port 0) listen config

When listen_port is 0 the OS assigns the port at bind time, so the
configured value can no longer be trusted for building takeover URLs.

- server: read listener.local_addr() after bind and propagate the
  actual port to the global proxy port, status, and ProxyServerInfo
- services: start the proxy before takeover when port is 0 so live
  configs get the real port instead of :0, and persist the resolved
  port back to the DB for DB-only URL paths; stop the pre-started
  server on any takeover failure
- claude_desktop: reject an unresolved :0 port instead of emitting a
  broken gateway URL
- build_proxy_urls: prefer the running server's port and error out if
  the port is still 0

Add tests for takeover with an ephemeral port and the claude_desktop
:0 rejection; switch existing codex takeover tests to an ephemeral
port for isolation.

src-tauri/src/proxy/server.rs

@@ -112,11 +112,15 @@ impl ProxyServer {
         let listener = tokio::net::TcpListener::bind(&addr)
             .await
             .map_err(|e| ProxyError::BindFailed(e.to_string()))?;
+        let local_addr = listener
+            .local_addr()
+            .map_err(|e| ProxyError::BindFailed(e.to_string()))?;
+        let actual_port = local_addr.port();
 
-        log::info!("[{}] 代理服务器启动于 {addr}", log_srv::STARTED);
+        log::info!("[{}] 代理服务器启动于 {local_addr}", log_srv::STARTED);
 
         // 更新全局代理端口，用于系统代理检测
-        crate::proxy::http_client::set_proxy_port(self.config.listen_port);
+        crate::proxy::http_client::set_proxy_port(actual_port);
 
         // 保存关闭句柄
         *self.shutdown_tx.write().await = Some(shutdown_tx);
@@ -125,7 +129,7 @@ impl ProxyServer {
         let mut status = self.state.status.write().await;
         status.running = true;
         status.address = self.config.listen_address.clone();
-        status.port = self.config.listen_port;
+        status.port = actual_port;
         drop(status);
 
         // 记录启动时间
@@ -213,7 +217,7 @@ impl ProxyServer {
 
         Ok(ProxyServerInfo {
             address: self.config.listen_address.clone(),
-            port: self.config.listen_port,
+            port: actual_port,
             started_at: chrono::Utc::now().to_rfc3339(),
         })
     }