feat: SteamOS theme from cachyos-vapor with its desktop layout, plus SteamOS desktop extras

- Install CachyOS's cachyos-vapor package instead of extracting Valve's
  steamdeck-kde-presets into ~/.local/share, and remove it again on disable
  if the wizard installed it.
- Apply the Vapor global theme with "Desktop and window layout"
  (lookandfeeltool --resetLayout): Steam Deck wallpaper, SteamOS panel and
  launcher icon. Layout files are backed up and restored, every key from
  Vapor's defaults goes through the undo journal.
- Restore the real previous color scheme (CachyOS's inline BreezeDark colors
  have no scheme name), and clear ColorScheme before plasma-apply-colorscheme,
  which otherwise skips an already named scheme.
- Register Vapor as the dark theme, so Plasma's Dark Mode toggle shows on.
- Drop the SteamOS desktop defaults (GTK, Konsole, gsettings, panel tweaks).
- New lib/steamos-extras.sh, downloaded from Valve's newest
  steamdeck-kde-presets (checksum verified) into /usr/local: Add to Steam,
  Nested Desktop, the gaming-return icon for Return to Gaming Mode, the
  Steam Keyboard window rule and an empty KWallet (only without a wallet).
- single_launcher: re-apply single user's launcher settings after the
  layout reset replaces the launcher.

setup-gamescope-boot.sh

@@ -16,7 +16,7 @@ set -uo pipefail
 
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 
-for lib in common state packages login-manager single-user steam-desktop steam-machine vapor-theme desktop-shortcut menu; do
+for lib in common state packages login-manager single-user steam-desktop steam-machine vapor-theme steamos-extras desktop-shortcut menu; do
     # shellcheck source=/dev/null
     source "$SCRIPT_DIR/lib/$lib.sh"
 done