fix: make setup auto-installs deps and handles container-owned dirs

- Add check-deps target: auto-installs python3, pip3, cryptography via
  apt (with pip fallback) before running setup_cell.py
- Add sudo chown in setup to reclaim config/data dirs if containers have
  taken ownership (e.g. re-running setup after make start)
- Pass PUID/PGID=$(id -u/g) to docker-compose so linuxserver/wireguard
  chowns its config dir to the host user instead of hardcoded UID 911

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

docker-compose.yml

@@ -164,8 +164,8 @@ services:
     container_name: cell-wireguard
     environment:
       - SERVERMODE=true
-      - PUID=911
-      - PGID=911
+      - PUID=${PUID:-1000}
+      - PGID=${PGID:-1000}
     ports:
       - "51820:51820/udp"
     volumes: