[fix](trx-frontend-http): update WSPR slot countdown timer

Wire the WSPR period label to a live 120-second slot countdown so
it no longer stays at the placeholder value.

Co-authored-by: Codex <codex@openai.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/plugins/wspr.js

@@ -1,13 +1,27 @@
 // --- WSPR Decoder Plugin (server-side decode) ---
 const wsprStatus = document.getElementById("wspr-status");
+const wsprPeriodEl = document.getElementById("wspr-period");
 const wsprMessagesEl = document.getElementById("wspr-messages");
 const WSPR_MAX_MESSAGES = 200;
+const WSPR_PERIOD_SECONDS = 120;
 
 function fmtWsprTime(tsMs) {
   if (!tsMs) return "--:--:--";
   return new Date(tsMs).toLocaleTimeString([], { hour: "2-digit", minute: "2-digit", second: "2-digit" });
 }
 
+function updateWsprPeriodTimer() {
+  if (!wsprPeriodEl) return;
+  const nowSec = Math.floor(Date.now() / 1000);
+  const remaining = WSPR_PERIOD_SECONDS - (nowSec % WSPR_PERIOD_SECONDS);
+  const mm = String(Math.floor(remaining / 60)).padStart(2, "0");
+  const ss = String(remaining % 60).padStart(2, "0");
+  wsprPeriodEl.textContent = `Next slot ${mm}:${ss}`;
+}
+
+updateWsprPeriodTimer();
+setInterval(updateWsprPeriodTimer, 500);
+
 function renderWsprRow(msg) {
   const row = document.createElement("div");
   row.className = "ft8-row";