Fix system audio command fallback

[akj]

Summary

• Keeps every available system audio command in priority order instead of selecting only the first command at backend construction time.
• Falls through to the next command when the primary system command fails, while capturing stderr for diagnostics.
• Skips commands that cannot handle the file format, e.g. aplay for non-WAV files.
• Preserves the existing per-command volume argv behavior for paplay, ffplay, afplay, and aplay.

Note

This PR is independent, but #23 fixes a pre-existing cgo malgo test abort that can affect full-suite CI on any branch. Merge #23 first if CI fails in internal/audio/malgo rather than in this changed package.

Test plan

• go test ./internal/audio
• go test ./internal/audio ./internal/config ./internal/soundpack ./internal/cli
• CGO_ENABLED=0 go test ./...

Full go test ./... on this branch hit the pre-existing malgo/PulseAudio abort fixed by #23.

[ctoth]

Thanks for working on this!