fix: Nemo terminal/keymap/vscode — alacritty default, CH+FR lock, code action

- dconf 00-cinnamon: add org.cinnamon+gnome.desktop.default-applications.terminal
  exec='alacritty' exec-arg='-e' so Nemo right-click → Open in Terminal works
- dconf locks/keyboard: lock input-sources/sources and terminal exec keys so
  csd-keyboard doesn't reset them to defaults on first session start
- build-live-iso.sh: add code-open wrapper script (/usr/local/bin/code-open)
  that finds 'code' in the live user's nix profile; add open-in-vscode
  Nemo action to /usr/share/nemo/actions/
- config/install.conf: add nixpkgs#vscode to NIX_USER_PACKAGES
- packages.live-desktop.list: add setxkbmap (needed by csd-keyboard to
  apply XKB keyboard layout)

config/install.conf

@@ -77,6 +77,7 @@ NIX_USER_PACKAGES=(
   "nixpkgs#localsend"
   "nixpkgs#google-chrome"
   "nixpkgs#mission-center"
+  "nixpkgs#vscode"
 )
 
 # ---------- Cinnamon customization ----------