fix(mibook): boot-menu terminal choice via specialisation

The previous startOnBoot approach booted mibook headless by default and
locked the machine out: it is WiFi-only with credentials in KWallet, so
with no desktop session NetworkManager never joins the network (no SSH),
and boot stalled on NetworkManager-wait-online with the tty1 prompt
buried under service logs.

Replace it with a NixOS specialisation that adds a separate 'terminal'
GRUB entry:
- default entry boots KDE as before (identical to baseline);
- 'terminal' entry boots multi-user.target with autologin for finn and a
  'desktop' command to start SDDM on demand.
Also disable NetworkManager-wait-online so boot never stalls on the
network. Revert the kde-desktop startOnBoot option.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CfozKLQdUh5TzqyjSigLUx

machines/mibook/configuration.nix

@@ -20,7 +20,6 @@
     useOSProber = true;
   };
 
-
   # Configure keymap in X11
   services.xserver.xkb = {
     layout = "de";
@@ -34,7 +33,6 @@
   services.printing.enable = true;
   nixpkgs.config.allowUnfree = true;
 
-
   hardware.nvidia.prime = {
     sync.enable = false;
 
@@ -44,6 +42,37 @@
 
   services.openssh.enable = true;
 
+  # Don't let boot stall waiting for a network that may never come up
+  # (WiFi credentials live in KWallet and need a desktop session), which
+  # otherwise hangs the terminal boot before the login prompt appears.
+  systemd.services.NetworkManager-wait-online.enable = false;
+
+  # Boot-time choice: the default GRUB entry boots straight into KDE.
+  # A separate "terminal" entry (a NixOS specialisation) boots to a text
+  # console with autologin, where you can work or run `desktop` to bring
+  # KDE up. Pick the entry you want in the GRUB menu at boot.
+  #
+  # NOTE: in terminal mode WiFi will not connect on its own (the password
+  # is stored per-user in KWallet). To reach the machine over SSH from
+  # terminal mode, first save the WiFi as a system connection in KDE:
+  # network settings -> your WiFi -> "All users may connect to this network".
+  specialisation.terminal.configuration = {
+    system.nixos.tags = [ "terminal" ];
+
+    # Boot to a text console. graphical.target is what pulls in the display
+    # manager (via its embedded Wants=display-manager.service), so defaulting
+    # to multi-user.target leaves SDDM installed but not started at boot.
+    systemd.defaultUnit = lib.mkForce "multi-user.target";
+
+    # Guarantee a usable shell on the console (no login prompt to hunt for).
+    services.getty.autologinUser = "finn";
+
+    # Bring the desktop up on demand from the terminal.
+    environment.systemPackages = [
+      (pkgs.writeShellScriptBin "desktop" "exec sudo systemctl start display-manager.service")
+    ];
+  };
+
   # KDE (PowerDevil) power settings: do nothing on lid close while on AC power.
   # Shipped as a system-wide default; KConfig cascades so a user's own
   # ~/.config/powerdevilrc will override this if present.