Phase 5a: persistent websocket with request ids and reconnect

Client (ts/network_ts.ts): one WebSocket per page. Commands queue and go
out one at a time with a client-chosen id; the response is matched on the
echoed id (or to the in-flight request for older servers). 12 s timeout
per request, exponential-backoff reconnect (0.5 s to 10 s), the in-flight
request is retried after a reconnect, and log polls are de-duplicated so
they cannot pile up behind a stalled connection. The old global flag that
silently dropped any request made while another was in flight is gone.

Server (src/webserver.go): the /data/ handler now serves any number of
commands on one connection (it used to break out of its loop after the
first reply without closing the socket, leaving it open and deaf; the old
client papered over that by opening a new socket per request). Connection
closed on exit, request id echoed in the response.

Test: TestWSServesMultipleCommandsPerConnection.

src/struct-webserver.go

@@ -5,6 +5,10 @@ type RequestStruct struct {
 	// Befehle an xTeVe
 	Cmd string `json:"cmd"`
 
+	// Client-chosen request id, echoed in the response so the web UI can
+	// match answers on a long-lived connection.
+	ID string `json:"id,omitempty"`
+
 	// Benutzer
 	DeleteUser bool           `json:"deleteUser,omitempty"`
 	UserData   map[string]any `json:"userData,omitempty"`
@@ -73,6 +77,8 @@ type RequestStruct struct {
 
 // ResponseStruct : Antworten an den Client (WEB)
 type ResponseStruct struct {
+	ID string `json:"id,omitempty"`
+
 	ClientInfo struct {
 		ARCH      string `json:"arch"`
 		Branch    string `json:"branch,omitempty"`