feat: Add var_gpu flag for GPU passthrough configuration

[MickLesk]

✍️ Description

Changes:

• Add var_gpu variable to CT scripts for explicit GPU control
• Remove hardcoded GPU_APPS list - GPU detection now uses var_gpu flag
• Add var_gpu to VAR_WHITELIST for persistence in default.vars and app.vars
• Add GPU Passthrough option (Step 19) to advanced_settings wizard (now 20 steps)
• Update documentation

Apps with var_gpu=yes (GPU enabled by default):

• Media: jellyfin, plex, emby, channels, ersatztv, tunarr, immich
• Transcoding: tdarr, unmanic, fileflows
• AI/ML: ollama, openwebui
• NVR: frigate

Usage:

• Disable GPU: var_gpu=no bash -c '$(curl -fsSL ...jellyfin.sh)'
• Enable GPU: var_gpu=yes bash -c '$(curl -fsSL ...debian.sh)'
• Via default.vars: echo 'var_gpu=yes' >> /usr/local/community-scripts/default.vars
• Via advanced settings wizard (Step 19)

✅ Prerequisites (X in brackets)

• Self-review completed – Code follows project standards.
• Tested thoroughly – Changes work as expected.
• No security risks – No hardcoded secrets, unnecessary privilege escalations, or permission issues.

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🛠️ Type of Change (X in brackets)

• 🐞 Bug fix – Resolves an issue without breaking functionality.
• ✨ New feature – Adds new, non-breaking functionality.
• 💥 Breaking change – Alters existing functionality in a way that may require updates.
• 🆕 New script – A fully functional and tested script or script set.
• 🌍 Website update – Changes to website-related JSON files or metadata.
• 🔧 Refactoring / Code Cleanup – Improves readability or maintainability without changing functionality.
• 📝 Documentation update – Changes to README, AppName.md, CONTRIBUTING.md, or other docs.