[feat](trx-frontend-http): centre the radio controls, split off the mode row

The controls every rig has — mode, wheel, tune step, transmit — now sit
as a centred block rather than packed against the left edge.

What the current mode adds moves out from among them: WFM's six controls
stretched the row sideways whenever it was active, pushing the wheel and
the step pickers off centre, and SAM did the same on a smaller scale.
They get a row of their own below a divider, which appears and leaves
with them — an empty one would still take a track and a gap in the tray
and draw its divider under controls it has nothing to do with.

Signed-off-by: Stan Grams <sjg@haxx.space>

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

@@ -278,9 +278,17 @@ input.status-input, select.status-input { width: 100%; padding: 0.45rem 0.5rem;
 .controls-row {
   display: flex;
   flex-wrap: wrap;
+  justify-content: center;
   gap: 1rem 1.25rem;
   align-items: flex-start;
 }
+/* Whatever the current mode adds — WFM's deemphasis and interference meters,
+   SAM's carrier sync — sits on its own line under the controls every rig has,
+   instead of stretching the row sideways when one mode happens to be active. */
+.controls-row-mode {
+  padding-top: 0.85rem;
+  border-top: 1px solid color-mix(in srgb, var(--border-light) 45%, transparent);
+}
 .controls-col {
   min-width: 0;
   display: flex;