[style](trx-frontend-http): make bookmark markers follow theme accent colour

Replace hardcoded amber values with var(--accent-yellow) throughout the
bookmark axis labels, so they automatically adapt to all UI themes.
Centre spans vertically with top:50%/translate(-50%,-50%) for equal
padding above and below. Canvas dashed line uses pal.waveformPeak
(already theme-aware) at 0.65 opacity.

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

@@ -4274,7 +4274,7 @@ function drawSpectrum(data) {
     : [];
   if (visBookmarks.length > 0) {
     ctx.save();
-    ctx.strokeStyle = "rgba(246,173,85,0.65)";
+    ctx.strokeStyle = pal.waveformPeak.replace(/[\d.]+\)$/, "0.65)");
     ctx.lineWidth = 1 * dpr;
     ctx.setLineDash([4 * dpr, 3 * dpr]);
     for (const bm of visBookmarks) {