[fix](trx-frontend-http): hover bookmark axis over waterfall/waveform seam

Switch #spectrum-bookmark-axis from position:relative (creates gap) to
position:absolute at top:var(--spectrum-plot-height) with
transform:translateY(-50%), so it floats centred on the boundary
without affecting layout. z-index:6 keeps it above the BW/freq
selector signal overlay (z-index 4).

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/style.css

@@ -1517,11 +1517,14 @@ button:focus-visible, input:focus-visible, select:focus-visible {
   font-weight: 700;
 }
 #spectrum-bookmark-axis {
-  position: relative;
-  z-index: 5;
+  position: absolute;
+  top: var(--spectrum-plot-height);
+  transform: translateY(-50%);
+  left: 0;
+  right: 0;
+  z-index: 6;
   height: 0;
   overflow: hidden;
-  width: 100%;
   font-size: 0.68rem;
   background: var(--bg-secondary);
   border-top: 1px solid var(--border);