Feature/advanced settings inherit app defaults

[MickLesk]

✍️ Description

1.) inherit app defaults in advanced settings wizard

• All var_* values from CT scripts now pre-populate wizard fields
• Added TUN/TAP support step (Step 19) with var_tun default
• Added Nesting support step (Step 20) with var_nesting default
• FUSE, GPU, Verbose now show '(App default: X)' hints
• Nesting feature now user-configurable (was hardcoded)
• MAX_STEP increased from 20 to 22
• All feature flags (FUSE, TUN, GPU, Nesting) saved to app defaults
• Summary shows all features including TUN and Nesting status

2.)
Advanced Settings Wizard (28 steps):

• Step 18: FUSE Support (inherits var_fuse)
• Step 19: TUN/TAP Support (inherits var_tun)
• Step 20: Nesting Support (inherits var_nesting)
• Step 21: GPU Passthrough (inherits var_gpu)
• Step 22: Keyctl Support (inherits var_keyctl)
• Step 23: APT Cacher Proxy (inherits var_apt_cacher/var_apt_cacher_ip)
• Step 24: Container Timezone (inherits var_timezone)
• Step 25: Container Protection (inherits var_protection)
• Step 26: Device Node Creation (inherits var_mknod)
• Step 27: Mount Filesystems (inherits var_mount_fs)
• Step 28: Verbose Mode & Confirmation

3.)
pre-fill timezone with host timezone in advanced settings

🔗 Related Issue

Fixes #

✅ 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.