feat(radio): answer the RTL-SDR card through the radio store

SdrCard calls LocalSdrControls through radio store actions instead of
DOM listeners, and renders its gain options from the receiver's own
R820T gain steps with page-language labels. The display and radio
stores share one owner list (createOwners): the newest connected owner
with an action answers it.

src/ui/stores/owners.test.mjs

@@ -0,0 +1,28 @@
+// src/ui/stores/owners.test.mjs — the newest connected owner with an action
+// answers it; detaching hands the action back to the one before.
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { createOwners } from './owners.js';
+
+test('the newest owner with an action answers it, and detach hands it back', () => {
+  const { connect, detach, act } = createOwners();
+  const calls = [];
+  const first = connect({
+    play: (n) => calls.push(['first.play', n]),
+    stop: () => calls.push(['first.stop']),
+  });
+  const second = connect({ play: (n) => calls.push(['second.play', n]) });
+  act('play', 1);
+  act('stop');
+  act('missing');
+  detach(second);
+  act('play', 2);
+  detach(second);
+  detach(first);
+  act('play', 3);
+  assert.deepEqual(calls, [
+    ['second.play', 1],
+    ['first.stop'],
+    ['first.play', 2],
+  ]);
+});