feat: new Antergos NeXT Minimal (Xfce) edition — branding, theme, and packaging

Introduces a lightweight Xfce-based variant of Antergos NeXT to work
around the 2 GB GitHub Releases limit for the full Plasma ISO.

New packages in antergos-packages:
- calamares-branding-antergos-next-minimal: Calamares branding variant
  that says "Minimal (Xfce)" with a notesqml page explaining why this
  edition exists (Plasma ISO exceeded GitHub's 2 GB limit).
- antergos-xfce-theme: GTK 2.0/3.0/3.20 + xfwm4 theme based on Numix,
  recolored to Antergos brand accent (#4A9EFF). Includes xfce-notify-4.0,
  metacity-1, openbox-3, and unity support directories.

packages.yaml updated with both new packages in the build list.

packages/calamares-branding-antergos-next-minimal/settings_offline.conf

@@ -0,0 +1,51 @@
+---
+modules-search: [ /usr/lib/calamares/modules, /etc/calamares/modules ]
+
+sequence:
+    - show:
+        - welcome
+        - locale
+        - keyboard
+        - partition
+        - users
+        - summary
+    - exec:
+        - partition
+        - mount
+        - unpackfs
+        - machineid
+        - fstab
+        - locale
+        - keyboard
+        - localecfg
+        - luksopenswaphookcfg
+        - luksbootkeyfile
+        - initcpiocfg
+        - initcpio
+        - users
+        - displaymanager
+        - networkcfg
+        - hwclock
+        - services-artix
+        - grubcfg
+        - bootloader
+        - postcfg
+        - umount
+    - show:
+        - finished
+
+branding: antergos-next-minimal
+
+prompt-install: false
+
+dont-chroot: false
+
+oem-setup: false
+
+disable-cancel: false
+
+disable-cancel-during-exec: false
+
+hide-back-and-next-during-exec: false
+
+quit-at-end: false