feat: branding DE/DM selectors, slideshow content redesign, layan wallpaper fix

calamares-branding-antergos-next:
- Replace packagechooser.conf (method: legacy, init selector) with
  packagechooser_de.conf (9 DEs, netinstall-add) and packagechooser_dm.conf
  (5 DMs, netinstall-select)
- Clean netinstall.yaml: remove Budgie, split i3/Sway, drop hidden groups
- Update settings_online.conf with packagechooser@de + @dm instances
- Redesign slideshow: 7 slides, accurate copy (Artix-based, not Arch),
  accent bar, better layout
- Bump pkgrel to 23

antergos-layan-theme:
- Fix plasma-org.kde.plasma.desktop-appletsrc video wallpaper path:
  Retro%20Sun%20WPE.mp4 -> antergos-wallpaper.mp4

packages/calamares-branding-antergos-next/packagechooser_dm.conf

@@ -0,0 +1,27 @@
+---
+mode: required
+method: netinstall-select
+labels:
+    step: "Display Manager"
+default: SDDM
+items:
+    - id: SDDM
+      name: "SDDM"
+      description: "QML-based display manager (KDE default)"
+      screenshot: ""
+    - id: LightDM
+      name: "LightDM"
+      description: "Cross-desktop display manager (GTK)"
+      screenshot: ""
+    - id: LXDM
+      name: "LXDM"
+      description: "Lightweight display manager (LXDE)"
+      screenshot: ""
+    - id: greetd
+      name: "greetd"
+      description: "Minimal display manager (recommended for COSMIC)"
+      screenshot: ""
+    - id: LY
+      name: "LY"
+      description: "Minimal TUI display manager"
+      screenshot: ""