Replace caelestia shell with Waybar and Walker

Remove caelestia-shell/cli inputs and prune corresponding flake.lock
entries. Update Hyprland keybindings to use walker, grimblast and
Waybar, use loginctl for locking and pkill/waybar for restart. Add
new home/system modules for waybar and walker and enable them in
host configs. Delete obsolete INSTALL-laptop.md.

home/system/walker/default.nix

@@ -0,0 +1,9 @@
+{pkgs, ...}: {
+  home.packages = with pkgs; [
+    walker
+  ];
+
+  wayland.windowManager.hyprland.settings.exec-once = [
+    "walker --gapplication-service"
+  ];
+}