[feat](trx-frontend-http): intercept freq/mode changes for virtual channels

When on a non-primary (virtual) channel, redirect freq and mode changes
to the channel metadata API instead of the server:

- vchan.js: add vchanIsOnVirtual(), vchanSetChannelFreq/Mode(); expose
  window.vchanInterceptMode() hook; wrap window.setRigFrequency so all
  callers (jog, freq input, bookmarks, spectrum click) are automatically
  redirected without modification
- app.js: check vchanInterceptMode() in applyModeFromPicker() before
  posting /set_mode
- bookmarks.js: check vchanInterceptMode() for mode in bmApply();
  setRigFrequency() redirect is automatic via the vchan.js wrapper;
  bandwidth and decoder toggles still apply regardless of channel

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

@@ -3119,6 +3119,10 @@ async function applyModeFromPicker() {
   modeEl.disabled = true;
   showHint("Setting mode…");
   try {
+    if (typeof vchanInterceptMode === "function" && await vchanInterceptMode(mode)) {
+      showHint("Channel mode set", 1500);
+      return;
+    }
     await postPath(`/set_mode?mode=${encodeURIComponent(mode)}`);
     showHint("Mode set", 1500);
     if (mode.toUpperCase() === "WFM") {