Fix: Use TZ from .env instead of hardcoded value

The timezone collected during docker-install.sh was being saved
to .env but docker-compose.yml had a hardcoded TZ=Europe/Berlin.

Changed to TZ=${TZ:-Europe/Berlin} to use .env value with fallback.

docker-compose.yml

@@ -26,7 +26,7 @@ services:
       # This ensures the slicer only needs to trust one CA certificate.
       - ./virtual_printer:/app/data/virtual_printer
     environment:
-      - TZ=Europe/Berlin
+      - TZ=${TZ:-Europe/Berlin}
       # Port BamBuddy runs on (default: 8000)
       # Usage: PORT=8080 docker compose up -d
       - PORT=${PORT:-8000}