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.
wl-clipboard is on PATH so the shell can pipe to and from the clipboard.

modules/desktop/clipboard.nix

@@ -0,0 +1,48 @@
+{
+  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} = {
+      # wl-copy and wl-paste on PATH, so the shell can pipe into and out of the
+      # clipboard.
+      # The watchers and picker above reach wl-clipboard by store path, so this
+      # is for interactive use alone.
+      home.packages = [ pkgs.wl-clipboard ];
+
+      # 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}"
+      ];
+    };
+  };
+}