feat: complete skylanix rebrand and preseed improvements

- Replace yunohost grub theme with skylanix theme
- Update isolinux menu colors to skylanix branding (#4490a5)
- Refactor preseed configuration with improved package management
- Move Firefox Mozilla installation to main preseed
- Consolidate user shell configuration and system setup
- Streamline late custom scripts for AMD and NVIDIA variants

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

files_to_inject/isolinux/menu.cfg

@@ -6,8 +6,8 @@ menu title skylanix installation
 menu background splash.png
 menu color title	* #FFFFFFFF *
 menu color border	* #00000000 #00000000 none
-menu color sel		* #ffffffff #76a1d0ff *
-menu color hotsel	1;7;37;40 #ffffffff #76a1d0ff *
+menu color sel		* #ffffffff #4490a5 *
+menu color hotsel	1;7;37;40 #ffffffff #4490a5 *
 menu color tabmsg	* #ffffffff #00000000 *
 menu color help		37;40 #ffdddd00 #00000000 none
 menu vshift 16