[feat](trx-frontend-http): vchan freq display sync, BW accent, scheduler multi-channel

Virtual channel display:
- vchan.js: wrap refreshFreqDisplay() so the main freq field always shows
  the active virtual channel's frequency instead of channel 0's; expose
  vchanSyncAccentUI() to add vchan-ch-active CSS class (colored border) to
  #freq and #spectrum-bw-input when on a non-primary channel
- style.css: --vchan-color (#38bdf8 sky-blue), .vchan-ch-active box-shadow,
  vchan-picker active button left-border accent

Scheduler multi-channel slots:
- scheduler.rs: add center_hz (Option<u64>) and bookmark_ids (Vec<String>)
  to ScheduleEntry; SchedulerStatus gains last_center_hz and
  last_bookmark_ids; background task sends SetCenterFreq before SetFreq
  when center_hz is set and records extra bookmark_ids in status
- scheduler.js: center-freq input and extra-channel bookmark picker (tag
  list with + / × buttons) in the add-entry form; extra channels shown in
  the entries table
- index.html: center freq field + extra bookmark picker widgets; table
  gains Center freq and Extra channels columns

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

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

@@ -90,6 +90,8 @@ function vchanRender() {
   addBtn.title = "Allocate new virtual channel at current frequency";
   addBtn.addEventListener("click", vchanAllocate);
   picker.appendChild(addBtn);
+
+  vchanSyncAccentUI();
 }
 
 async function vchanAllocate() {
@@ -170,7 +172,7 @@ function vchanApplyCapabilities(caps) {
 }
 
 // ---------------------------------------------------------------------------
-// Freq / mode interception
+// Freq / mode interception + UI accent
 // ---------------------------------------------------------------------------
 
 // Returns true when the active channel is a non-primary (virtual) channel.
@@ -179,6 +181,40 @@ function vchanIsOnVirtual() {
   return vchanActiveId !== vchanChannels[0].id;
 }
 
+function vchanActiveChannel() {
+  return vchanChannels.find(c => c.id === vchanActiveId) || null;
+}
+
+// Update the main freq input to show the virtual channel's frequency.
+function vchanUpdateFreqDisplay() {
+  const ch = vchanActiveChannel();
+  if (!ch) return;
+  const el = document.getElementById("freq");
+  if (!el) return;
+  if (typeof formatFreqForStep === "function" && typeof jogUnit !== "undefined") {
+    el.value = formatFreqForStep(ch.freq_hz, jogUnit);
+  } else {
+    el.value = (ch.freq_hz / 1e6).toFixed(6).replace(/\.?0+$/, "");
+  }
+}
+
+// Add / remove the vchan accent class from the freq and BW inputs.
+function vchanSyncAccentUI() {
+  const onVirtual = vchanIsOnVirtual();
+  const freqEl = document.getElementById("freq");
+  const bwEl   = document.getElementById("spectrum-bw-input");
+  if (freqEl) freqEl.classList.toggle("vchan-ch-active", onVirtual);
+  if (bwEl)   bwEl.classList.toggle("vchan-ch-active", onVirtual);
+  if (onVirtual) {
+    vchanUpdateFreqDisplay();
+  } else if (typeof _origRefreshFreqDisplay === "function") {
+    _origRefreshFreqDisplay();
+  }
+}
+
+// Saved reference to the original refreshFreqDisplay from app.js.
+let _origRefreshFreqDisplay = null;
+
 async function vchanSetChannelFreq(freqHz) {
   if (!vchanRigId || !vchanActiveId) return;
   try {
@@ -235,3 +271,17 @@ window.vchanInterceptMode = async function(mode) {
     if (typeof _orig === "function") return _orig(freqHz);
   };
 })();
+
+// Wrap refreshFreqDisplay so the main freq field stays in sync with the
+// active virtual channel's frequency (SSE rig-state updates would otherwise
+// constantly overwrite it with channel 0's freq).
+(function() {
+  _origRefreshFreqDisplay = window.refreshFreqDisplay;
+  window.refreshFreqDisplay = function() {
+    if (vchanIsOnVirtual()) {
+      vchanUpdateFreqDisplay();
+      return;
+    }
+    if (typeof _origRefreshFreqDisplay === "function") _origRefreshFreqDisplay();
+  };
+})();