[fix](trx-frontend-http): prevent SSE from snapping freq back during optimistic update

When the user changes frequency, applyLocalTunedFrequency sets lastFreqHz
optimistically. But the SSE state stream could push back the old server
frequency before set_freq completes, causing the marker to snap back then
forward. Add a sequence-guarded _freqOptimisticHz that suppresses stale
SSE frequency updates while a set_freq is in flight.

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

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

@@ -338,6 +338,11 @@ async function bmApply(bm) {
       }
     }
     if (typeof applyLocalTunedFrequency === "function") {
+      // Set optimistic guard before applying so SSE cannot snap back.
+      if (typeof _freqOptimisticSeq !== "undefined") {
+        ++_freqOptimisticSeq;
+        _freqOptimisticHz = bm.freq_hz;
+      }
       applyLocalTunedFrequency(bm.freq_hz);
     }
     if (typeof scheduleSpectrumDraw === "function" && typeof lastSpectrumData !== "undefined" && lastSpectrumData) {