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/packages.list

@@ -110,10 +110,12 @@ gvfs-smb
 file-roller
 gnome-keyring
 seahorse
-network-manager-applet
 blueman
 bluez
 
+# --- bluetooth audio ---
+bluez-alsa
+
 # --- display manager ---
 # niri can be launched directly via TTY (`niri-session`) or via a wayland-aware
 # greeter. We use greetd + tuigreet — lighter than lightdm under wayland.
@@ -170,9 +172,6 @@ system-config-printer
 sane
 simple-scan
 
-# --- bluetooth audio ---
-bluez-alsa
-
 # --- backups / snapshots ---
 timeshift
 grub-btrfs