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

@@ -0,0 +1,52 @@
+title-color: "white"
+title-text: ""
+title-font: "Sans Regular 16"
+desktop-color: "#44a0a5"
+message-color: "white"
+message-bg-color: "#44a0a5"
+terminal-font: "Sans Regular 12"
+
++ boot_menu {
+  left = 20%
+  width = 60%
+  top = 260
+  height = 200
+  item_font = "Sans Regular 12"
+  item_color = "white"
+  selected_item_color = "black"
+  selected_item_pixmap_style = "hl_*.png"
+  item_height = 20
+  item_padding = 15
+  item_spacing = 10
+}
+
++ image {
+  top = 20
+  left = 50%-123
+  width = 246
+  height = 246
+  file = "logo.png"
+}
+
++ vbox {
+  top = 100%-60
+  left = 10%
+  + hbox {
+    top = 0
+    left = 20%
+    + label {text = "Enter: " font = "Sans 10" color = "white" align = "left"}
+    + label {text = "Select             " font = "Sans 10" color = "#c0c0c0" align = "left"}
+    + label {text = "           " font = "Sans 10" color = "white" align = "left"}
+    + label {text = "Esc:   " font = "Sans 10" color = "white" align = "left"}
+    + label {text = "Back up one level  " font = "Sans 10" color = "#c0c0c0" align = "left"}
+  }
+  + hbox {
+    top = 0
+    left = 20%
+    + label {text = "E:     " font = "Sans 10" color = "white" align = "left"}
+    + label {text = "Edit Selection     " font = "Sans 10" color = "#c0c0c0" align = "left"}
+    + label {text = "           " font = "Sans 10" color = "white" align = "left"}
+    + label {text = "C:     " font = "Sans 10" color = "white" align = "left"}
+    + label {text = "GRUB Command line" font = "Sans 10" color = "#c0c0c0" align = "left"}
+  }  
+}