fix: deploy-disk.sh --boot argument parsing + installer combo ids

The installer failed with:

    deploy-disk.sh /dev/vda ... workstation --boot
    error: --boot must be bios|efi (got --boot)

Two bugs:

1. deploy-disk.sh treated $4 as the bare boot value, but every caller
   (installer, build-image.sh, install.sh) passes "--boot <val>", so $4
   was the literal "--boot". It now parses "--boot <bios|efi>", accepts a
   bare positional too, and falls back to the BOOT build variable. This
   also fixes build-image.sh / install.sh, which had the same mismatch.

2. installer/arcline-installer built the boot ComboBox with append(id,text)
   arguments swapped, so get_active_id() returned "UEFI (grub-efi-amd64)"
   instead of "efi". Fixed the id/text order.

installer/arcline-installer

@@ -125,8 +125,9 @@ class Installer(Gtk.Window):
 
         # boot mode
         self.boot_combo = Gtk.ComboBoxText()
-        for label, val in (("BIOS (grub-pc)", "bios"), ("UEFI (grub-efi-amd64)", "efi")):
-            self.boot_combo.append(label, val)
+        # ComboBoxText.append(id, text) — id is the value we pass to deploy
+        for val, label in (("bios", "BIOS (grub-pc)"), ("efi", "UEFI (grub-efi-amd64)")):
+            self.boot_combo.append(val, label)
         self.boot_combo.set_active(0)
         row_boot = Gtk.Box(orientation=Gtk.Orientation.HORIZONTAL, spacing=8)
         row_boot.pack_start(Gtk.Label(label="Boot mode:"), False, False, 0)