feat: add PairDrop file sharing service (drop.pivoine.art)

Added PairDrop stack for peer-to-peer file sharing:
- WebRTC-based direct file transfers between devices
- No file size limits or server storage
- End-to-end encrypted transfers
- Local network auto-discovery
- Cross-platform support (desktop, mobile, tablets)
- Progressive Web App installable on mobile
- Rate limiting enabled for security

PairDrop provides secure, private file sharing without uploading
files to any server - all transfers happen directly between devices.

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

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

arty.yml

@@ -124,6 +124,10 @@ envs:
     JELLY_TRAEFIK_ENABLED: true
     JELLY_COMPOSE_PROJECT_NAME: jelly
     JELLY_TRAEFIK_HOST: jelly.pivoine.art
+    # PairDrop
+    DROP_TRAEFIK_ENABLED: true
+    DROP_COMPOSE_PROJECT_NAME: drop
+    DROP_TRAEFIK_HOST: drop.pivoine.art
     # Proxy
     PROXY_COMPOSE_PROJECT_NAME: proxy
     PROXY_DOCKER_IMAGE: traefik:latest