[feat](trx-frontend-http): GPU-composited CSS overlay for instant freq/BW updates

Replace synchronous drawSignalOverlay() calls in freq/BW change handlers
with lightweight CSS div elements repositioned via transform: translateX().
This is GPU-composited with zero layout/paint cost, making frequency and
bandwidth changes appear instantaneous. The full WebGL overlay catches up
on the next requestAnimationFrame.

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/index.html

@@ -160,6 +160,9 @@
         <span id="signal-split-value" aria-live="polite">50/50</span>
       </div>
       <canvas id="signal-overlay-canvas" aria-hidden="true"></canvas>
+      <div id="fast-freq-marker" aria-hidden="true"></div>
+      <div id="fast-bw-left" aria-hidden="true"></div>
+      <div id="fast-bw-right" aria-hidden="true"></div>
     </div>
     <div class="status">
       <div class="full-row freq-row">