feat: default volume configuration

[aaron-r-campbell]

Added default_volume option to App section of config. Addresses #109

Configuration Details

• default_volume = -1 (default): The player controls initial volume (current behavior). In my case mpv is persisting my previous session volume somehow, which I prefer over a set default.
• default_volume = 0-100: Sets a specific default volume on startup.
• default_volume > 100 or < 0 (except -1): Values are automatically clamped to the 0-100 range.

Volume Adjustment Behavior

1. Adjustments now snap to the nearest multiple of volumeStep if not already aligned (e.g., 43 goes up to 45, down to 40)
2. If already a multiple, behaves as before

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Additional Notes

I noticed some redundancy between for example mpvClient.CurrentVolume and player.GetVolume, which could be considered in a future refactor / code cleanup.

[MattiaPun]

Hi Aaron, thank you for this PR. It will be available in the next release