ImageUpdateService previously discovered stacks by iterating Docker
containers, which meant stacks without containers (e.g. after
docker compose down) were silently excluded from update checks.
Switch to a hybrid discovery approach: enumerate stacks from the
filesystem via FileSystemService.getStacks(), parse compose files
for image refs with .env variable resolution, then augment with
container-based image discovery for running stacks.
Also cleans up stale stack_update_status entries when stacks are
deleted or no longer exist on disk, and replaces the plain
update-available tooltip with an animated cursor follow pattern.
Anso