Add custom notification sound support

[Copilot]

Adds ability for users to select custom audio files as notification sounds. Currently limited to system sounds only.

Backend

• Integrated rodio (v0.21.1) for cross-platform audio playback
• Added play_custom_audio_file() with async execution to prevent UI blocking
• Modified play_notification_sound() to accept optional path with fallback to system sounds
• Added test_custom_sound() command for preview functionality

Frontend

• File picker dialog with filter for common audio formats (MP3, WAV, OGG, FLAC, M4A, AAC, WMA)
• UI controls: select, change, clear buttons with file path display
• Test button routes to custom or system sound based on configuration
• Settings persistence via existing storage mechanism
• Full i18n support (zh-CN, en-US)

Implementation Notes

Audio stream lifecycle managed via sleep_until_end() to ensure complete playback before resource cleanup. UI toggle uses direct DOM manipulation matching existing pattern from resetOnIdleToggle.

fn play_custom_audio_file(file_path: &str) -> Result<(), String> {
    let (_stream, stream_handle) = OutputStream::try_default()?;
    let sink = Sink::try_new(&stream_handle)?;
    let file = File::open(file_path)?;
    let source = Decoder::new(BufReader::new(file))?;
    sink.append(source);
    sink.sleep_until_end();  // Keeps _stream alive
    Ok(())
}

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