feat: add host-side auto-updater for zero-touch API container rebuilds

When the in-container self-updater downloads a new version, it writes
trigger.json. The new systemd path unit watches for this file and runs
dashcaddy-update.sh, which backs up current API files, copies the new
ones, rebuilds the container, verifies health, and writes result.json.
Automatic rollback on build or health check failure.

Also fixes undefined `isWindows` variable in self-updater.js and adds
DASHCADDY_HOST_UPDATES_DIR env var to the installer's docker-compose
template for correct container-to-host path translation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

dashcaddy-installer/install.sh

@@ -661,6 +661,7 @@ services:
       - DASHCADDY_UPDATE_URL=https://get.dashcaddy.net/release
       - DASHCADDY_MIRROR_URL=https://get2.dashcaddy.net/release
       - DASHCADDY_UPDATES_DIR=/app/updates
+      - DASHCADDY_HOST_UPDATES_DIR=/opt/dashcaddy/updates
       - DASHCADDY_API_SOURCE_DIR=${API_DIR}
       - DASHCADDY_FRONTEND_DIR=/app/dashboard
     extra_hosts: