feat(grub): rebuild on every wallpaper change via udt-grub

GRUB's background is a wallpaper snapshot, but only install.sh rebuilt it,
so a wallpaper change left the /boot copy stale with no hint. udt-grub now
holds the build-and-print logic; install.sh runs it in the foreground and
udt-accent starts it detached next to udt-homepage. A stale installed theme
gets a notification carrying the root copy command. A flock serialises the
two runs install.sh triggers, since build swaps dist/udt in by rename.

AGENTS.md: udt-accent writes seven outputs (SDDM was missing), and names
the two detached helpers.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 6 insertions, 3 deletions

diff --git a/bin/udt-accent b/bin/udt-accent
index 72d3a64..1027fab 100755
--- a/bin/udt-accent
+++ b/bin/udt-accent
@@ -477,9 +477,12 @@ def main(image):
 
     # The dashboard host. Detached, because wallp runs this synchronously and
     # an unreachable host would hold the wallpaper change for the scp timeout.
-    subprocess.Popen([Path(__file__).resolve().parent / "udt-homepage"],
-                     stdin=subprocess.DEVNULL, stdout=subprocess.DEVNULL,
-                     stderr=subprocess.DEVNULL, start_new_session=True)
+    # GRUB's background is a wallpaper snapshot. Detached for the same reason;
+    # it only rebuilds and notifies, since the copy into /boot needs root.
+    for helper in ("udt-homepage", "udt-grub"):
+        subprocess.Popen([Path(__file__).resolve().parent / helper],
+                         stdin=subprocess.DEVNULL, stdout=subprocess.DEVNULL,
+                         stderr=subprocess.DEVNULL, start_new_session=True)
 
     before, after = hue_neighbours(name)
     print(f"{name} {ACCENTS[name]} (border: {before} .. {name} .. {after})")