[feat](trx-frontend-http): frequency-ordered z-index and hover-to-front for RDS overlay layers

Each RDS PS overlay item (position: absolute within the shared #rds-ps-overlay
container) now receives a z-index derived from its channel frequency: items are
sorted by freq_hz ascending so higher-frequency layers sit on top of
lower-frequency ones by default.

Hovering any layer temporarily assigns it the maximum z-index (entry count + 10)
to bring it to the front; mouseleave restores the frequency-derived default
stored in data-default-z.

Also reverts the incorrectly applied vchan picker layer changes from the
previous commit.

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

@@ -262,7 +262,6 @@
             <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>