[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/frontend/tests/browser-smoke.mjs

@@ -49,6 +49,23 @@ try {
   assert.equal(modeAfter.active, target, "the clicked mode is not the marked one");
   assert.ok(modeAfter.selectWidth <= 2, `the hidden select still occupies ${modeAfter.selectWidth}px`);
 
+  // Mode-specific controls live on their own row, which has to leave with them:
+  // an empty one would still take a track and a gap in the tray and draw its
+  // divider under the controls every rig has.
+  const modeRowState = async () => page.evaluate(() => {
+    const row = document.getElementById("mode-controls-row");
+    return { display: getComputedStyle(row).display, height: Math.round(row.getBoundingClientRect().height) };
+  });
+  await page.locator('#mode-picker button[data-mode="WFM"]').click();
+  await page.waitForTimeout(250);
+  const withWfm = await modeRowState();
+  assert.notEqual(withWfm.display, "none", "WFM controls did not bring their row up");
+  assert.ok(withWfm.height > 0, `WFM row has no height (${withWfm.height}px)`);
+  await page.locator('#mode-picker button[data-mode="FM"]').click();
+  await page.waitForTimeout(250);
+  const withoutWfm = await modeRowState();
+  assert.equal(withoutWfm.display, "none", "the mode row stayed behind with nothing in it");
+
   // Scheduler controls read left to right: step, hand back, then the entry on
   // air. The separator is drawn by the current-entry block, so it can only sit
   // in the right place if that block is last.