Add Docker/Compose setup and fix subpath routing for /toroid deployment

- Add Dockerfile, .dockerignore, and docker-compose.yml to containerize the app
- Add toroid.service systemd unit (uses docker compose)
- Mount presets/ as a bind volume for persistence outside the container
- Fix all fetch() calls in templates to use relative paths (no leading /)
  so they resolve correctly when served under /toroid/ via nginx proxy

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

toroid.service

@@ -0,0 +1,14 @@
+[Unit]
+Description=Toroid Transformer Designer
+After=docker.service
+Requires=docker.service
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+WorkingDirectory=/home/brent/um-apps/toroid
+ExecStart=/usr/bin/docker compose up -d
+ExecStop=/usr/bin/docker compose down
+
+[Install]
+WantedBy=multi-user.target