fix: add GRUB efiInstallAsRemovable as fallback bootloader

modules/common/system.nix

@@ -3,8 +3,19 @@
 {
   system.stateVersion = "25.11";
 
-  boot.loader.systemd-boot.enable = true;
+  # Try UEFI first, fall back to GRUB for legacy BIOS
-  boot.loader.efi.canTouchEfiVariables = true;
+  boot.loader = {
+    systemd-boot.enable = true;
+    efi.canTouchEfiVariables = true;
+
+    # GRUB as fallback for legacy/CSM boot
+    grub = {
+      enable = true;
+      device = "nodev";
+      efiSupport = true;
+      efiInstallAsRemovable = true;
+    };
+  };
 
   services.xserver.xkb.layout = "uk";
 }