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/boot/grub/theme/skylanix

@@ -1,9 +1,9 @@
 title-color: "white"
 title-text: ""
 title-font: "Sans Regular 16"
-desktop-color: "black"
+desktop-color: "#44a0a5"
 message-color: "white"
-message-bg-color: "black"
+message-bg-color: "#44a0a5"
 terminal-font: "Sans Regular 12"
 
 + boot_menu {