refactor(frontend): isolate audio meters and apply settings on save

vocero-frontend/src/realtime/controller.test.ts

@@ -87,6 +87,32 @@ async function start() {
   return client;
 }
 describe("voice application lifecycle", () => {
+  it("notifies only meter subscribers when microphone level changes", async () => {
+    const client = await start();
+    const app = vi.fn();
+    const meter = vi.fn();
+    controller.subscribe(app);
+    controller.subscribeLevel(meter);
+    const snapshot = controller.getSnapshot();
+    client.emit("level", 0.2);
+    expect(controller.getLevel()).toBe(0.2);
+    expect(meter).toHaveBeenCalledOnce();
+    expect(app).not.toHaveBeenCalled();
+    expect(controller.getSnapshot()).toBe(snapshot);
+  });
+  it("saves only changed preferences and keeps noise calibration out of session updates", async () => {
+    const client = await start();
+    const save = vi.spyOn(Storage.prototype, "setItem");
+    client.updateSession.mockClear();
+    controller.changeSettings({ noiseGate: -35 });
+    expect(save).toHaveBeenCalledExactlyOnceWith("s2s.ws.noiseGate", "-35");
+    expect(client.setGate).toHaveBeenCalledExactlyOnceWith({ enabled: true, thresholdDb: -35 });
+    expect(client.updateSession).not.toHaveBeenCalled();
+    controller.changeSettings({ voice: "Serena", instructions: "New instructions" });
+    expect(client.updateSession).toHaveBeenCalledOnce();
+    controller.changeSettings({ voice: "Serena", instructions: "New instructions" });
+    expect(client.updateSession).toHaveBeenCalledOnce();
+  });
   it("renders configuration and starts without waiting for optional reports", async () => {
     const original = fetchMock.getMockImplementation();
     if (!original) {