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

modules/environments/kde-desktop/default.nix

@@ -11,37 +11,17 @@ in
 {
   options.my.profiles.kde-desktop = with lib; {
     enable = mkEnableOption "KDE Desktop Environment";
-    startOnBoot = mkOption {
-      type = types.bool;
-      default = true;
-      description = ''
-        Whether the display manager (SDDM) starts automatically at boot.
-        When false, the system boots to a text console and KDE can be
-        launched on demand with the `desktop` command.
-      '';
-    };
   };
 
-  config = lib.mkIf cfg.enable (lib.mkMerge [
-    {
-      services = {
-        displayManager.sddm.enable = true;
-        displayManager.sddm.wayland.enable = true;
-        desktopManager.plasma6.enable = true;
-      };
-      users.users.finn.packages = with pkgs; [
-        # Programms can be added here...
-        numix-icon-theme
-      ];
-    }
-    (lib.mkIf (!cfg.startOnBoot) {
-      # Boot to a text console; SDDM stays installed but is not pulled in
-      # by any boot target. Launch KDE on demand with `desktop`.
-      systemd.services.display-manager.wantedBy = lib.mkForce [ ];
-      systemd.defaultUnit = lib.mkForce "multi-user.target";
-      environment.systemPackages = [
-        (pkgs.writeShellScriptBin "desktop" "exec sudo systemctl start display-manager.service")
-      ];
-    })
-  ]);
-}
+  config = lib.mkIf cfg.enable {
+    services = {
+      displayManager.sddm.enable = true;
+      displayManager.sddm.wayland.enable = true;
+      desktopManager.plasma6.enable = true;
+    };
+    users.users.finn.packages = with pkgs; [
+      # Programms can be added here...
+      numix-icon-theme
+    ];
+  };
+}