Add player backend install hints

[pitill0]

Summary

Improve player backend diagnostics by showing actionable install hints when supported playback backends are missing.

Changes

• Add install hints for mpv, ffplay, mpg123 and ogg123.
• Extend fluxtuner --list-players to show install hints for missing backends.
• Show a compact install help message when player selection fails because no suitable backend is available.
• Add tests for backend install hints.
• Document player backend troubleshooting in the usage guide.

Validation

• python -m fluxtuner --list-players
• ruff check .
• python -m pytest tests/test_player_listing.py
• python -m pytest
• python -m bandit -r fluxtuner
• pip-audit --local