Add .env support for configurable Name and Slogan in preview cards

NAME and SLOGAN are read from .env via dotenv and injected into the
LinkedIn preview card template at startup. Avatar initials are auto-
generated from the first letters of NAME. Works identically with
npm start and docker compose (via env_file).

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

package.json

@@ -7,6 +7,7 @@
     "start": "node server.js"
   },
   "dependencies": {
+    "dotenv": "^17.4.1",
     "express": "^4.18.2"
   }
 }