feat(sub): expose live online status and add ?format=info endpoint

Custom subscription templates only received the lastOnline timestamp, so
template authors had to fake an online indicator by comparing it against
the current time, and the page was a one-shot server render with no way
to refresh usage without reloading the whole HTML.

The template context (and window.__SUB_PAGE_DATA__) now carries isOnline,
computed from the panel's own online-client tracking (local xray plus
remote nodes) at render time. The subscription URL also answers
?format=info with the page view-model as JSON — minus the links, with
emails deduplicated — so templates can poll live status cheaply. The
shared view-model construction moved into buildSubPageData/subPageContext
so the HTML page, the SPA payload and the info JSON cannot drift apart.

Also documents the previously injected but undocumented announce
template variable.

internal/sub/service.go

@@ -2454,6 +2454,7 @@ type PageData struct {
 	BasePath      string
 	SId           string
 	Enabled       bool
+	IsOnline      bool
 	Download      string
 	Upload        string
 	Total         string
@@ -2618,6 +2619,7 @@ func (s *SubService) BuildPageData(subId string, hostHeader string, traffic xray
 		BasePath:      basePath,
 		SId:           subId,
 		Enabled:       traffic.Enable,
+		IsOnline:      subIsOnline(emails, s.inboundService.GetOnlineClients()),
 		Download:      download,
 		Upload:        upload,
 		Total:         total,
@@ -2639,6 +2641,22 @@ func (s *SubService) BuildPageData(subId string, hostHeader string, traffic xray
 	}
 }
 
+func subIsOnline(subEmails, onlineEmails []string) bool {
+	if len(subEmails) == 0 || len(onlineEmails) == 0 {
+		return false
+	}
+	onlineSet := make(map[string]struct{}, len(onlineEmails))
+	for _, email := range onlineEmails {
+		onlineSet[email] = struct{}{}
+	}
+	for _, email := range subEmails {
+		if _, online := onlineSet[email]; online {
+			return true
+		}
+	}
+	return false
+}
+
 func getHostFromXFH(s string) (string, error) {
 	if strings.Contains(s, ":") {
 		realHost, _, err := net.SplitHostPort(s)