Fix nix single-user mode for live ISO and installer

- Store ownership: chown -R 1000:1000 at Docker build time (not runtime)
  so the live user can create lock files without flooding the tmpfs overlay
- nix.conf: add build-users-group= to force single-user mode and avoid
  daemon connection attempts (xbps nix-daemon v2.30.2 incompatible with
  pre-baked nix v2.34.6)
- profile.d: export NIX_REMOTE=local and NIXPKGS_ALLOW_UNFREE=1; wrap nix()
  to append --impure so flake installs work without extra flags
- Skel: add ~/.config/nixpkgs/config.nix with allowUnfree=true
- postinstall.sh: fix daemon socket path (/nix/var/nix/...), write
  ~/.config/nixpkgs/config.nix for installed user
- first-login.sh: add NIX_REMOTE=local alongside NIXPKGS_ALLOW_UNFREE=1
- Remove nix-daemon from live ISO services (wrong version for pre-baked client)
- Misc: bluetooth group, package list reorg, skip vscode install for niri profile

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config/profiles/mainline-niri/customizations/niri.sh

@@ -47,10 +47,8 @@ prefer-no-csd
 
 spawn-at-startup "swaybg" "-i" "/usr/share/backgrounds/void-installer/pxfuel.jpg"
 spawn-at-startup "mako"
-spawn-at-startup "nm-applet" "--indicator"
-spawn-at-startup "blueman-applet"
 spawn-at-startup "/usr/libexec/polkit-gnome-authentication-agent-1"
-spawn-at-startup "noctalia-shell"
+spawn-at-startup "sh" "-c" "i=0; while [ \$i -lt 30 ] && ! dbus-send --system --print-reply --dest=org.freedesktop.DBus /org/freedesktop/DBus org.freedesktop.DBus.GetNameOwner string:org.bluez >/dev/null 2>&1; do sleep 1; i=\$((i+1)); done; exec noctalia-shell"
 
 cursor {
     xcursor-theme "Bibata-Modern-Ice"