feat(desktop): add clipboard history (task 0027)

Add a clipboard module to the desktop group, enabled through the
aggregator. Home-manager's services.cliphist runs the text and image
watchers as systemd user services bound to the graphical-session
target, and a rofi-dmenu picker recalls history on Super+Shift+V.

modules/desktop/clipboard.nix

@@ -0,0 +1,42 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+# Clipboard history: cliphist records every copy, picked back through rofi.
+let
+  cfg = config.modules.desktop.clipboard;
+  user = config.user.name;
+
+  cliphist = "${pkgs.cliphist}/bin/cliphist";
+  rofi = "${pkgs.rofi}/bin/rofi";
+  wl-copy = "${pkgs.wl-clipboard}/bin/wl-copy";
+
+  # The picker reuses the themed rofi, so history looks like every other menu
+  # the launcher drives.
+  # decode is needed because list emits id-prefixed lines rather than the copied
+  # bytes, so the chosen id has to be resolved back before it can be re-copied.
+  picker = pkgs.writeShellScript "clipboard-picker" ''
+    ${cliphist} list \
+      | ${rofi} -dmenu -i -p Clipboard \
+      | ${cliphist} decode \
+      | ${wl-copy}
+  '';
+in
+{
+  options.modules.desktop.clipboard.enable = lib.mkEnableOption "cliphist clipboard history";
+
+  config = lib.mkIf cfg.enable {
+    home-manager.users.${user} = {
+      # Two watchers record text and images to history, bound to the graphical
+      # session so uwsm starts and stops them with it.
+      services.cliphist.enable = true;
+
+      # $mod is defined by the compositor config these binds share.
+      wayland.windowManager.hyprland.settings.bind = [
+        "$mod SHIFT, V, exec, ${picker}"
+      ];
+    };
+  };
+}

modules/desktop/desktop.nix

@@ -9,6 +9,7 @@ in
   # Each piece is turned on at default priority, so a host can still override
   # any one of them while the single flag above enables the whole desktop.
   config = lib.mkIf cfg.enable {
+    modules.desktop.clipboard.enable = lib.mkDefault true;
     modules.desktop.hyprland.enable = lib.mkDefault true;
     modules.desktop.hyprlock.enable = lib.mkDefault true;
     modules.desktop.hypridle.enable = lib.mkDefault true;