feat: add smart audio copy and audio mix presets for configurable 5.1 downmixing

public/index.html

@@ -548,6 +548,20 @@
                   <option value="low">Low (Fastest)</option>
                 </select>
               </div>
+              <div class="setting-item">
+                <div class="setting-info" style="flex: 1;">
+                  <span class="setting-label">Audio Mix Preset</span>
+                  <span class="setting-hint">How multi-channel audio (5.1/7.1) is mixed down to stereo for browser
+                    playback.</span>
+                </div>
+                <select id="setting-audio-mix" class="source-select" style="width: auto; min-width: 200px;">
+                  <option value="auto">Auto (Smart Copy/ITU)</option>
+                  <option value="itu">ITU-R BS.775 (Balanced)</option>
+                  <option value="night">Night Mode (Dialogue Boost)</option>
+                  <option value="cinematic">Cinematic (Wide)</option>
+                  <option value="passthrough">Passthrough (Copy)</option>
+                </select>
+              </div>
             </div>
 
             <div class="settings-section">