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>

compose.yaml

@@ -13,6 +13,7 @@ include:
   - vert/compose.yaml
   - paint/compose.yaml
   - jelly/compose.yaml
+  - drop/compose.yaml
   - restic/compose.yaml
   - umami/compose.yaml
   - sablier/compose.yaml