[style](trx-frontend-http): move bandplan strip from bottom to top of spectrum

Repositions the bandplan rendering above the waterfall instead of below
the spectrum waveform. Updates both the WebGL draw position (y=0) and
the CSS overlay positioning (top:0) for the label layer.

https://claude.ai/code/session_01Bt6iUi6Pc1v7yvLffEjweJ
Signed-off-by: Claude <noreply@anthropic.com>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/style.css

@@ -3303,10 +3303,10 @@ button:focus-visible, input:focus-visible, select:focus-visible {
 
 /* ── WebGL overlay mode ── When the spectrum canvas is visible the coloured
    segments are rendered via WebGL.  The DOM strip repositions itself as a
-   transparent label overlay at the bottom of the spectrum canvas. */
+   transparent label overlay at the top of the spectrum canvas. */
 #spectrum-bandplan-strip.bp-webgl {
   position: absolute;
-  top: calc(var(--spectrum-plot-height) - 18px);
+  top: 0;
   left: 0;
   right: 0;
   background: transparent;