Add wallpaper switch

hm-modules/theme/set-wallpaper.sh

@@ -0,0 +1,27 @@
+# shellcheck shell=bash
+# set-wallpaper — bind the runtime wallpaper symlink to $1 and restart hyprpaper
+# so it picks up the new image. hyprpaper's config (managed by Home Manager)
+# points at the symlink, so we never write hyprpaper config ourselves.
+#
+# Env (injected by the Nix wrapper):
+#   wallpaperSymlink  path to the symlink hyprpaper's `wallpaper.path` reads
+# Arg: image path
+
+: "${wallpaperSymlink:?set-wallpaper: wallpaperSymlink not set}"
+set -euo pipefail
+
+target="${1:?usage: set-wallpaper <image-path>}"
+[ -f "$target" ] || { log_error "not a file: $target"; exit 1; }
+
+mkdir -p "$(dirname "$wallpaperSymlink")"
+ln -sfn "$target" "$wallpaperSymlink"
+
+# Apply live by (re)starting the user hyprpaper service. `systemctl restart`
+# starts the unit even if it is inactive — important because a freshly-added
+# hyprpaper service won't be running until the first switch. Tolerate the
+# service being absent (e.g. on a host without hyprpaper).
+if command -v systemctl >/dev/null 2>&1 && systemctl --user restart hyprpaper 2>/dev/null; then
+  log_info "applied $(basename "$target") (hyprpaper started)"
+else
+  log_warn "could not start hyprpaper; wallpaper symlink set but not displayed"
+fi