Migrate to hm managed theme switch

hm-modules/theme/theme-switch.sh

@@ -0,0 +1,77 @@
+# shellcheck shell=bash
+# theme-switch — activate a pre-built Home Manager specialisation.
+#
+# Expects two variables to be injected by the Nix wrapper:
+#   baseVariant        the variant that is the generation itself (not a
+#                      specialisation), activated via `$gen/activate`.
+#   availableVariants  space-separated list of all variant names (help text).
+#
+# Standalone usage for testing:
+#   baseVariant=gruvbox-dark \
+#   availableVariants="gruvbox-dark gruvbox-light catppuccin-dark catppuccin-light" \
+#   bash theme-switch.sh <variant>
+
+: "${baseVariant:?theme-switch: baseVariant not set}"
+: "${availableVariants:?theme-switch: availableVariants not set}"
+set -euo pipefail
+
+variant="${1:-}"
+
+if [ -z "$variant" ]; then
+  echo "Usage: theme-switch <variant>"
+  echo "Available: ${availableVariants}"
+  exit 1
+fi
+
+# Resolve the Home Manager *base* generation — the one built by NixOS that
+# actually contains the `specialisation/` directory holding every variant's
+# activation script.
+#
+# IMPORTANT: do NOT use `current-home` / the HM profile as the primary
+# source. Activating a specialisation repoints those to the specialisation's
+# *leaf* generation, which has no `specialisation/` of its own — so after
+# switching once, every other variant (and the base) would become unfindable.
+# The NixOS-managed `home-manager-«user».service` unit references the base
+# generation and only changes on a rebuild.
+user="${USER:-$(id -un)}"
+gen=""
+if unit="$(systemctl cat "home-manager-${user}.service" 2>/dev/null)"; then
+  gen="$(printf '%s\n' "$unit" | grep -oE '/nix/store/[0-9a-z]+-home-manager-generation' | head -n1)"
+fi
+
+# Fallbacks for standalone Home Manager, where the profile symlink / gcroot
+# points at the base generation (not a specialisation leaf).
+for candidate in \
+  "${HOME}/.local/state/home-manager/gcroots/current-home" \
+  "/nix/var/nix/profiles/per-user/${user}/home-manager" \
+  "${HOME}/.local/state/home-manager/home-manager-generation"
+do
+  [ -n "$gen" ] && break
+  if [ -e "$candidate" ]; then
+    gen="$(readlink -e "$candidate")" || gen=""
+  fi
+done
+
+if [ -z "$gen" ]; then
+  echo "error: could not resolve Home Manager base generation" >&2
+  exit 1
+fi
+
+case "$variant" in
+  "${baseVariant}")
+    # Base variant is the generation itself, not a specialisation.
+    activate="$gen/activate"
+    ;;
+  *)
+    activate="$gen/specialisation/$variant/activate"
+    ;;
+esac
+
+if [ ! -x "$activate" ]; then
+  echo "error: variant '$variant' not found at $activate" >&2
+  echo "Available: ${availableVariants}"
+  exit 1
+fi
+
+echo "Switching theme to $variant..."
+exec "$activate"