[feat](trx-frontend-http): move bookmark axis between waterfall and waveform

Relocate #spectrum-bookmark-axis from inside .spectrum-wrap to the
flex gap between .overview-strip (waterfall) and #spectrum-panel
(waveform). Give it z-index:5 so labels sit above the signal-overlay-
canvas (BW/freq selector, z-index:4). Drop the now-unneeded
#spectrum-freq-axis.bm-axis-open border-radius hack and the
corresponding JS class toggle.

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

@@ -78,12 +78,12 @@
         <div id="rds-ps-overlay" aria-live="polite" aria-label="RDS station name"></div>
         <div id="aprs-bar-overlay" aria-live="polite" aria-label="Recent APRS frames"></div>
       </div>
+      <div id="spectrum-bookmark-axis"></div>
       <div id="spectrum-panel" style="display:none;">
         <div class="spectrum-wrap">
           <canvas id="spectrum-canvas"></canvas>
           <div id="spectrum-tooltip"></div>
           <div id="spectrum-freq-axis"></div>
-          <div id="spectrum-bookmark-axis"></div>
         </div>
         <div id="spectrum-controls">
           <div id="spectrum-bw-row">