fix(openflare): refresh WS read deadline on JSON pong; capture frpc stderr

- read_pump: 收到客户端 JSON {"type":"pong"} 时调用 conn.SetReadDeadline 刷新
  服务端读超时。修复前，服务端仅在 WebSocket 协议层 Pong 帧时刷新 deadline，
  而客户端使用 JSON 应用层 pong 回复，导致服务端 90s 后超时关闭连接，
  客户端收到 EOF 并触发无限重连循环。在 Cloudflare 代理场景下，
  100s 空闲超时进一步加剧了此问题。

- frpc/manager: 捕获 frpc 子进程 stderr 并在进程异常退出时
  将其内容记录到结构化日志 stderr 字段，便于诊断 exit status 1 的
  具体原因（如配置格式错误、Auth Token 失败、relay 服务端不可达等）。

internal/apps/flared/frpc/manager.go

@@ -192,6 +192,8 @@ func (m *Manager) restartProcess(ctx context.Context, relayID string, configPath
 			ensureNoOrphanProcess(pidPath)
 
 			cmd := exec.CommandContext(procCtx, m.cfg.FrpcPath, "-c", configPath) //nolint:gosec // FrpcPath and configPath are managed trusted locations
+			var stderrBuf bytes.Buffer
+			cmd.Stderr = &stderrBuf
 
 			m.mu.Lock()
 			proc.Cmd = cmd
@@ -213,10 +215,15 @@ func (m *Manager) restartProcess(ctx context.Context, relayID string, configPath
 				return
 			}
 
+			stderrOutput := strings.TrimSpace(stderrBuf.String())
 			if err != nil {
 				proc.LastError = err.Error()
 				proc.Status = "error"
-				slog.Error("frpc process exited unexpectedly", "relay_id", relayID, "error", err)
+				if stderrOutput != "" {
+					slog.Error("frpc process exited unexpectedly", "relay_id", relayID, "error", err, "stderr", stderrOutput)
+				} else {
+					slog.Error("frpc process exited unexpectedly", "relay_id", relayID, "error", err)
+				}
 			} else {
 				proc.Status = "stopped"
 				proc.LastError = "exited unexpectedly with code 0"