[feat](trx-frontend-http): set the squelch on the spectrum, in dB

The threshold is in dB, and since the squelch fix that is the scale the
spectrum axis and the S-meter are labelled in — so the control belongs
on the plot, at the level it gates.  A dashed line spans the spectrum at
its threshold with a grip that reads it out, dragged like the bandwidth
edges, green while the signal is above it and amber while it gates.
Arrow keys move it a dB at a time for anyone not using a mouse.

The audio row keeps a compact version: the dB, an indicator lit from the
same meter the DSP compares against, Auto, and an enable toggle that no
longer doubles as the level.  The slider ran 0-100% over that dB range,
which gave the operator a number with nothing on screen to relate it to,
and zero meant "disabled", so turning the squelch off to listen threw
the threshold away.  Auto now says which level it picked.

Two things the browser could only show once it was on the plot: the grip
landed underneath the split control at the right edge, which swallowed
its pointer, and dragging to the foot of the axis hid the line — and the
grip with it — instead of pinning it where it could be dragged back.

The fixture could not exercise any of this: /audio answered 404, which
hides the audio row and the control inside it, and the status carried no
filter block, which is what tells the client the rig has a squelch at
all.  Both now look like an SDR, and the spectrum test drives the line.

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

src/trx-client/trx-frontend/trx-frontend-http/frontend/tests/web-fixture.mjs

@@ -111,7 +111,17 @@ export async function startWebFixture({
         signal_meter: spectrum,
       },
     },
-    status: { freq: { hz: 100_000_000 }, mode: "FM", tx_en: false, vfo: null, tx: null, rx: null, lock: null },
+    status: { freq: { hz: 100_000_000 }, mode: "FM", tx_en: false, vfo: null, tx: null, rx: { sig: -70 }, lock: null },
+    // Reported only by SDR backends, and what makes the client show the
+    // squelch control at all.
+    filter: spectrum
+      ? {
+        bandwidth_hz: 12_000,
+        sdr_squelch_enabled: false,
+        sdr_squelch_threshold_db: -95,
+        sdr_agc_enabled: false,
+      }
+      : null,
     band: null,
     enabled: true,
     initialized: true,
@@ -184,8 +194,11 @@ export async function startWebFixture({
       response.end(JSON.stringify(jsonRoutes.get(url.pathname)));
       return;
     }
+    // 200 means "audio is configured": the client hides the whole audio row —
+    // and the squelch control with it — when this 404s.
     if (url.pathname === "/audio") {
-      response.writeHead(404).end();
+      response.writeHead(200, { "content-type": "application/json" });
+      response.end(JSON.stringify({ sample_rate: 48_000, channels: 1 }));
       return;
     }
     if (spectrum && url.pathname === "/spectrum") {