feat(desktop): add screenshot capture (task 0028)

Add a screenshot module wiring grimblast (grim + slurp) through the satty
annotation editor, enabled by the desktop aggregator. Region, active-window,
and full-screen captures each open in satty and, on confirm, land in both the
clipboard and a dated file under ~/Pictures/Screenshots.

Bound to Print / Shift+Print / Ctrl+Print rather than the spec's Super+L
family, whose keys task 0021 already holds for hjkl focus and window movement.

modules/desktop/desktop.nix

@@ -16,6 +16,7 @@ in
     modules.desktop.login.enable = lib.mkDefault true;
     modules.desktop.mako.enable = lib.mkDefault true;
     modules.desktop.rofi.enable = lib.mkDefault true;
+    modules.desktop.screenshot.enable = lib.mkDefault true;
     modules.desktop.terminal.enable = lib.mkDefault true;
     modules.desktop.theming.enable = lib.mkDefault true;
     modules.desktop.waybar.enable = lib.mkDefault true;

modules/desktop/screenshot.nix

@@ -0,0 +1,50 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+# Keyboard-driven screenshots that open in satty for annotation, then land in
+# both the clipboard and a dated file.
+let
+  cfg = config.modules.desktop.screenshot;
+  user = config.user.name;
+
+  grimblast = "${pkgs.grimblast}/bin/grimblast";
+  satty = "${pkgs.satty}/bin/satty";
+  wl-copy = "${pkgs.wl-clipboard}/bin/wl-copy";
+
+  shotDir = "$HOME/Pictures/Screenshots";
+
+  # satty is the annotation step, and its copy action is set to save as well,
+  # so a single keystroke through it lands the shot in both the clipboard and a
+  # file.
+  capture =
+    target:
+    pkgs.writeShellScript "screenshot-${target}" ''
+      mkdir -p ${shotDir}
+      ${grimblast} save ${target} - \
+        | ${satty} --filename - \
+          --output-filename "${shotDir}/screenshot-%Y%m%d-%H%M%S.png" \
+          --copy-command ${wl-copy} \
+          --save-after-copy \
+          --early-exit \
+          --actions-on-enter save-to-clipboard
+    '';
+in
+{
+  options.modules.desktop.screenshot.enable = lib.mkEnableOption "screenshots via grimblast and satty";
+
+  config = lib.mkIf cfg.enable {
+    home-manager.users.${user} = {
+      # Print with plain/Shift/Ctrl for region/window/full.
+      # Super+L, the spec's chosen key, is already the hjkl focus and movement
+      # bind, so screenshots take the Print key instead.
+      wayland.windowManager.hyprland.settings.bind = [
+        ", Print, exec, ${capture "area"}"
+        "SHIFT, Print, exec, ${capture "active"}"
+        "CTRL, Print, exec, ${capture "screen"}"
+      ];
+    };
+  };
+}