feat: add WebRTC STUN server configuration to PairDrop

Added RTC configuration with Google's public STUN servers to enable
peer-to-peer connections across different networks (e.g., WiFi to
cellular data).

Changes:
- Created drop/rtc_config.json with 5 Google STUN servers
- Updated drop/compose.yaml to mount RTC config file
- Added RTC_CONFIG environment variable pointing to config file

This should resolve connectivity issues when devices are on different
networks or behind NAT.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

drop/compose.yaml

@@ -3,12 +3,15 @@ services:
     image: lscr.io/linuxserver/pairdrop:latest
     container_name: ${DROP_COMPOSE_PROJECT_NAME}_app
     restart: unless-stopped
+    volumes:
+      - ./drop/rtc_config.json:/config/rtc_config.json:ro
     environment:
       PUID: 1000
       PGID: 1000
       TZ: ${TIMEZONE:-Europe/Berlin}
       RATE_LIMIT: true
       WS_FALLBACK: false
+      RTC_CONFIG: /config/rtc_config.json
     networks:
       - compose_network
     labels: