feat(desktop): add the rofi launcher (task 0024)

Add a rofi module to the desktop group, enabled by the aggregator.
It combines drun and window into one combi prompt, with calc and emoji
as further modes, bound on Super+R. A session menu built on the same
themed rofi is bound on Super+Shift+X, and every rofi -dmenu call
inherits the Stylix theme, so later utility menus reuse it for free.

Also give layer surfaces their own near-instant fade-in (layersIn and
fadeLayersIn), so the launcher appears quicker than the windows it
sits over.

modules/desktop/rofi.nix

@@ -0,0 +1,63 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+# rofi as the desktop launcher and dmenu-style menu frontend.
+let
+  cfg = config.modules.desktop.rofi;
+  user = config.user.name;
+
+  # rofi carrying the plugins that back its calc and emoji modes.
+  rofiPkg = pkgs.rofi.override {
+    plugins = [
+      pkgs.rofi-calc
+      pkgs.rofi-emoji
+    ];
+  };
+
+  # A session menu built on the same themed rofi, so the power actions look
+  # like every other menu the launcher drives.
+  powerMenu = pkgs.writeShellScript "rofi-power-menu" ''
+    chosen=$(printf '%s\n' Lock Suspend 'Log out' Reboot 'Shut down' \
+      | ${rofiPkg}/bin/rofi -dmenu -i -p Power)
+    case "$chosen" in
+      Lock) ${pkgs.systemd}/bin/loginctl lock-session ;;
+      Suspend) ${pkgs.systemd}/bin/systemctl suspend ;;
+      'Log out') ${pkgs.systemd}/bin/loginctl terminate-session "$XDG_SESSION_ID" ;;
+      Reboot) ${pkgs.systemd}/bin/systemctl reboot ;;
+      'Shut down') ${pkgs.systemd}/bin/systemctl poweroff ;;
+    esac
+  '';
+in
+{
+  options.modules.desktop.rofi.enable = lib.mkEnableOption "the rofi launcher";
+
+  config = lib.mkIf cfg.enable {
+    home-manager.users.${user} = {
+      # No theme is set here: Stylix's rofi target supplies the Nord one.
+      programs.rofi = {
+        enable = true;
+        package = rofiPkg;
+
+        # drun and window merge into one prompt through combi.
+        # run is left out because bare $PATH binaries are terminal tools, not
+        # windowed apps to launch.
+        # calc and emoji are the other two modes the same window tabs between.
+        extraConfig = {
+          modi = "combi,calc,emoji";
+          combi-modi = "drun,window";
+          show-icons = true;
+        };
+      };
+
+      # The launcher and the session menu, kept next to the tool they drive.
+      # $mod is defined by the compositor config these binds share.
+      wayland.windowManager.hyprland.settings.bind = [
+        "$mod, R, exec, ${rofiPkg}/bin/rofi -show combi"
+        "$mod SHIFT, X, exec, ${powerMenu}"
+      ];
+    };
+  };
+}