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.

lib/desktop-shortcut.sh

@@ -43,9 +43,10 @@ EOF
 
     # SteamOS itself uses Valve's "gaming-return" icon (Steam logo with a
     # return arrow; the Deck-style arrow is only used on Steam Deck
-    # hardware). It comes with the Vapor theme; without it, use Steam's icon.
+    # hardware). The theme installs it (lib/steamos-extras.sh) and updates
+    # the shortcut via set_shortcut_icon; without it, use Steam's icon.
     local shortcut_icon="steam"
-    [[ -f "$user_home/.local/share/icons/hicolor/scalable/actions/gaming-return.svg" ]] &&
+    [[ -f /usr/local/share/icons/hicolor/scalable/actions/gaming-return.svg ]] &&
         shortcut_icon="gaming-return"
 
     # 2. Generate the .desktop shortcut with instant session switcher strings
@@ -69,6 +70,14 @@ EOF
     ok "Desktop shortcut created successfully."
 }
 
+set_shortcut_icon() {
+    # set_shortcut_icon <icon>: update an existing shortcut, if any.
+    local shortcut
+    shortcut="$(xdg-user-dir DESKTOP 2>/dev/null || echo "$HOME/Desktop")/Return to Gaming Mode.desktop"
+    [[ -f "$shortcut" ]] && sed -i "s/^Icon=.*/Icon=$1/" "$shortcut"
+    return 0
+}
+
 remove_desktop_shortcut() {
     local desktop_dir
     desktop_dir="$(xdg-user-dir DESKTOP 2>/dev/null || echo "$HOME/Desktop")"