[feat](trx-frontend-http): sync mode picker to active virtual channel

- Add vchanSyncModeDisplay() in vchan.js; called from vchanSyncAccentUI()
  and vchanSubscribe() so the mode picker always reflects the active
  virtual channel's mode on switch and on channel-list refresh
- Guard the rig-state mode picker update in render() so it is skipped
  when vchanIsOnVirtual() is true, preventing primary-channel mode from
  overwriting the virtual channel selection

Note: per-channel audio and decoder output require server-side protocol
changes (separate Opus streams per virtual channel) and are not yet
implemented.

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/app.js

@@ -2445,7 +2445,12 @@ function render(update) {
   if (update.status && update.status.mode) {
     const mode = normalizeMode(update.status.mode);
     const modeUpper = mode ? mode.toUpperCase() : "";
-    modeEl.value = modeUpper;
+    // When subscribed to a virtual channel the mode picker must reflect
+    // that channel's mode, not the primary rig mode.  Skip the update here;
+    // vchan.js will apply the correct mode via vchanSyncModeDisplay().
+    if (typeof vchanIsOnVirtual !== "function" || !vchanIsOnVirtual()) {
+      modeEl.value = modeUpper;
+    }
     if (modeUpper === "WFM" && lastModeName !== "WFM") {
       setJogDivisor(10);
       resetRdsDisplay();