feat: add file-based admin settings panel at /admin

Settings saved in the panel are written to data/settings.json and
overlaid onto process.env, taking precedence over environment
variables and applying immediately without restart. Enable by setting
ADMIN_PASSWORD; on serverless platforms without persistent disk the
panel degrades to read-only.

docker-compose.yml

@@ -11,6 +11,9 @@ services:
         # - NEXT_PUBLIC_BASE_PATH=/nextaidrawio
     ports: ["3000:3000"]
     env_file: .env
+    volumes:
+      # Persists admin panel settings (data/settings.json)
+      - ./data:/app/data
     # environment:
     #   # For subdirectory deployment, uncomment and set your path:
     #   NEXT_PUBLIC_BASE_PATH: /nextaidrawio