feat(desktop): Print-key screenshots via grim/slurp

Bare Print grabs the whole screen to a timestamped PNG under
~/Pictures/Screenshots; $mod selects a region (slurp), SHIFT sends to the
clipboard instead. A mako toast confirms each capture. Adds grim/slurp and
enables xdg.userDirs for the Pictures target.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/home.nix

@@ -25,6 +25,31 @@ let
       | ${pkgs.pinentry-qt}/bin/pinentry-qt \
       | sed -n 's/^D //p'
   '';
+
+  # Screenshot helper for the Print-key binds below. Two axes, chosen by args:
+  # scope ($1 = full | region, the latter an interactive slurp selection) and
+  # destination ($2 = file | clip). Files are timestamped PNGs under
+  # ~/Pictures/Screenshots; clip copies to the Wayland clipboard. A mako toast
+  # confirms each grab. slurp exits non-zero on Escape, which aborts here.
+  screenshot = pkgs.writeShellScript "screenshot" ''
+    set -eu
+    if [ "$1" = region ]; then
+      geom=$(${pkgs.slurp}/bin/slurp) || exit 1
+      grab() { ${pkgs.grim}/bin/grim -g "$geom" "$@"; }
+    else
+      grab() { ${pkgs.grim}/bin/grim "$@"; }
+    fi
+    if [ "$2" = clip ]; then
+      grab - | ${pkgs.wl-clipboard}/bin/wl-copy
+      ${pkgs.libnotify}/bin/notify-send "Screenshot" "Copied to clipboard"
+    else
+      dir="${config.xdg.userDirs.pictures}/Screenshots"
+      mkdir -p "$dir"
+      file="$dir/$(date +%Y-%m-%d-%H%M%S).png"
+      grab "$file"
+      ${pkgs.libnotify}/bin/notify-send "Screenshot" "Saved $file"
+    fi
+  '';
 in
 {
   imports = [ ./gpg.nix ./media.nix ];
@@ -75,8 +100,20 @@ in
     nodejs
     highlight   # `ccat` alias: syntax-highlighted cat
     libreoffice
+    grim        # screenshot capture (also used by the Print-key binds)
+    slurp       # interactive region selector for grim
   ];
 
+  # Canonical XDG user dirs. Only Pictures is actually used (screenshot target,
+  # file-picker default). createDirectories stays off so the rest aren't
+  # littered into $HOME — they're merely declared in ~/.config/user-dirs.dirs,
+  # created lazily by whatever app wants them.
+  xdg.userDirs = {
+    enable = true;
+    createDirectories = false;
+    pictures = "${config.home.homeDirectory}/Pictures";
+  };
+
   # ---- Per-tool config files, routed into XDG dirs ---------------------
   xdg.configFile = {
     "python/startup".source = ./assets/python-startup.py;
@@ -374,6 +411,12 @@ in
         ",	XF86AudioMute,		exec,	wpctl set-mute @DEFAULT_AUDIO_SINK@ toggle"
         ",	XF86AudioRaiseVolume,	exec,	wpctl set-volume @DEFAULT_AUDIO_SINK@ 3%+"
         ",	XF86AudioLowerVolume,	exec,	wpctl set-volume @DEFAULT_AUDIO_SINK@ 3%-"
+        # Screenshots. $mod = pick a region (slurp), SHIFT = to clipboard;
+        # bare Print grabs the whole screen to a file (the common case).
+        ",		Print,	exec,	${screenshot} full file"
+        "$mod,		Print,	exec,	${screenshot} region file"
+        "SHIFT,		Print,	exec,	${screenshot} full clip"
+        "$mod SHIFT,	Print,	exec,	${screenshot} region clip"
       ];
 
       exec-once = [