fix(command): avoid command-string injection in show-text and out-of-range in cycle-audio

[Noethix55555]

Two fixes in Command.cs:

ShowText injection: ShowText built a raw mpv_command_string by concatenating the caller-supplied text directly. A double-quote in a media track title (e.g. He said hello) breaks the command. Fix: use Player.CommandV(show-text, ...) so each argument is a separate C string with no shell quoting needed.

CycleAudio out-of-range: CycleAudio always called tracks[aid - 1], but aid can be 0 when audio is disabled, or can exceed tracks.Length when external audio files have non-contiguous track IDs. Either case throws IndexOutOfRangeException. Fix: guard with if (aid < 1 || aid > tracks.Length) return before indexing.