fix(denoise-tester): play models in both ears; match the call's DeepFilterNet strength

- Recorded clips often decode as stereo (Chrome Opus/WebM). The models
  only process channel 0, and the default channel mode ignores
  channelCount, so RNNoise/Speex played the cleaned audio in the LEFT EAR
  ONLY. Downmix to mono before the models (as Lotus Call does, #24) and
  pin the RNNoise/Speex/gate nodes to explicit mono. Verified in Chromium
  with a stereo speech clip through RNNoise: before L 16.1 / R 0.0, after
  L 16.0 / R 15.9.
- DeepFilterNet ran at attenuation limit 80 here vs 60 in calls; both
  muted the background outright, which sounds like the mic cutting out
  at word ends. Use 25, matching element-call#65.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/features/settings/general/DenoiseTester.tsx

@@ -6,6 +6,7 @@ import {
   DenoiseNode,
   buildGateNode,
   buildModelNode,
+  monoDownmix,
   readDb,
   sampleRateFor,
 } from '../../../utils/denoisePipeline';
@@ -139,7 +140,7 @@ export function DenoiseTester({ model, useGate, gateThreshold, nativeNS }: Denoi
       inAnalyser.fftSize = 1024;
       source.connect(inAnalyser);
 
-      let head: AudioNode = source;
+      let head: AudioNode = source.connect(monoDownmix(ctx));
       let gate: AudioWorkletNode | null = null;
       if (useGate) {
         gate = await buildGateNode(ctx, gateThreshold);
@@ -308,10 +309,11 @@ export function DenoiseTester({ model, useGate, gateThreshold, nativeNS }: Denoi
       const ctx = new AudioContext({ sampleRate: sampleRateFor(playModel ?? 'rnnoise') });
       const source = ctx.createBufferSource();
       source.buffer = clip;
+      const mono = source.connect(monoDownmix(ctx));
       let playGate: AudioWorkletNode | null = null;
       let playModelNode: DenoiseNode | null = null;
       if (playModel) {
-        let head: AudioNode = source;
+        let head: AudioNode = mono;
         if (useGate) {
           playGate = await buildGateNode(ctx, gateThreshold);
           head.connect(playGate);
@@ -321,7 +323,7 @@ export function DenoiseTester({ model, useGate, gateThreshold, nativeNS }: Denoi
         head.connect(playModelNode.node);
         playModelNode.node.connect(ctx.destination);
       } else {
-        source.connect(ctx.destination);
+        mono.connect(ctx.destination);
       }
       // Superseded while the WASM/worklet loaded (another Play, a Stop, or the
       // panel unmounted)? Tear down this now-orphaned graph instead of storing