feat: add audio channel configuration for MPV player

- Add audio channels setting (auto/mono/stereo) to MPV properties
- Implement UI controls in settings panel and player right controls
- Enable immediate MPV property application when setting changes
- Update default playback type to LOCAL and sample rate to 48kHz

src/renderer/features/settings/components/playback/mpv-settings.tsx

@@ -32,6 +32,8 @@ export const getMpvSetting = (
     value: any,
 ) => {
     switch (key) {
+        case 'audioChannels':
+            return { 'audio-channels': value };
         case 'audioExclusiveMode':
             return { 'audio-exclusive': value || 'no' };
         case 'audioSampleRateHz':
@@ -53,6 +55,7 @@ export const getMpvSetting = (
 
 export const getMpvProperties = (settings: SettingsState['playback']['mpvProperties']) => {
     const properties: Record<string, any> = {
+        'audio-channels': settings.audioChannels || 'stereo',
         'audio-exclusive': settings.audioExclusiveMode || 'no',
         'audio-samplerate':
             settings.audioSampleRateHz === 0 ? undefined : settings.audioSampleRateHz,
@@ -271,6 +274,22 @@ export const MpvSettings = () => {
             isHidden: settings.type !== PlaybackType.LOCAL,
             title: t('setting.gaplessAudio', { postProcess: 'sentenceCase' }),
         },
+        {
+            control: (
+                <Select
+                    data={[
+                        { label: 'Mono', value: 'mono' },
+                        { label: 'Stereo', value: 'stereo' },
+                    ]}
+                    defaultValue={settings.mpvProperties.audioChannels || 'stereo'}
+                    onChange={(e) => handleSetMpvProperty('audioChannels', e)}
+                />
+            ),
+            description:
+                'Select the audio channel mode. Stereo is the default, mono downmixes to a single channel.',
+            isHidden: settings.type !== PlaybackType.LOCAL,
+            title: 'Audio Channels',
+        },
         {
             control: (
                 <NumberInput