feat(http-frontend): move spectrum above controls, add zoom/pan/tooltip

Spectrum panel is now placed above the freq/mode/jog row, spanning the
full card width. Key improvements:

- Scroll wheel zooms in/out at the cursor position (up to 64x); double-
  click resets to full bandwidth view.
- Mouse drag pans the visible window; click-to-tune is suppressed when a
  drag has occurred.
- Touch pinch-to-zoom and single-finger drag-to-pan supported.
- Hover tooltip shows the frequency under the cursor, formatted to the
  currently selected unit (MHz/kHz/Hz, matching the jog-step selection).
- Frequency axis labels update to reflect the zoomed visible range.
- Canvas height increased to 160 px; axis bar styled with card bg.
- A small hint line below the panel explains the controls.

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

@@ -58,6 +58,14 @@
       <div id="loading-sub" style="color:#9aa4b5;"></div>
     </div>
     <div id="content" style="display:none;">
+    <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>
+      <div id="spectrum-hint">Scroll to zoom &middot; Drag to pan &middot; Double-click to reset</div>
+    </div>
     <div class="status">
       <div class="full-row freq-row">
         <div class="inline freq-inline">
@@ -158,13 +166,6 @@
           </label>
         </div>
       </div>
-      <div id="spectrum-panel" style="display:none;">
-        <div class="label"><span>Spectrum</span></div>
-        <div class="spectrum-wrap">
-          <canvas id="spectrum-canvas"></canvas>
-          <div id="spectrum-freq-axis"></div>
-        </div>
-      </div>
     </div>
     <div class="full-row label-below-row" id="audio-row">
       <div class="label"><span>Audio</span></div>