[feat](trx-frontend-http): add frequency layers display for virtual channels

Render virtual channels as absolutely-positioned layer strips inside a
shared relative container (#vchan-freq-layers). Layers are sorted by
frequency ascending so higher-frequency channels receive a higher z-index
and sit on top by default. Hovering any layer temporarily assigns it the
maximum z-index to bring it to the front; leaving restores the original
stacking order. Each layer is offset by 11 px vertically so all channels
remain visible as a staggered card stack.

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/index.html

@@ -262,6 +262,7 @@
             <div class="full-row label-below-row" id="vchan-row" style="display:none;">
               <div class="label"><span>Channels</span></div>
               <div class="vchan-picker" id="vchan-picker"></div>
+              <div class="vchan-freq-layers" id="vchan-freq-layers"></div>
             </div>
             <div class="full-row label-below-row">
               <div class="label"><span>Signal</span></div>