docs: add PairDrop configuration to CLAUDE.md

Documented the PairDrop service with WebRTC STUN configuration:
- Service description and features
- RTC_CONFIG configuration with Google STUN servers
- Usage instructions for cross-network file sharing
- Technical details about WebRTC and NAT traversal

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

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

CLAUDE.md

@@ -296,6 +296,39 @@ Access https://paint.pivoine.art to use the image editor. All editing happens in
 
 **Note**: miniPaint is stateless and doesn't require backups as no data is persisted.
 
+### PairDrop (drop/compose.yaml)
+PairDrop peer-to-peer file sharing service:
+- **pairdrop**: PairDrop app (linuxserver/pairdrop image) exposed at `drop.pivoine.art:3000`
+  - Browser-based peer-to-peer file sharing
+  - WebRTC-based direct device-to-device transfers
+  - No server-side storage - files transfer directly between devices
+  - Works across different networks using STUN servers
+  - No account required - devices are discovered automatically
+  - Supports sharing files, text, and clipboard content
+  - Mobile and desktop browser support
+
+**Configuration**:
+- **RTC_CONFIG**: WebRTC configuration file at `/rtc_config.json`
+  - Configures STUN servers for NAT traversal
+  - Uses Google's public STUN servers for cross-network connectivity
+  - Enables peer connections between devices on different networks (WiFi to cellular, etc.)
+- **RATE_LIMIT**: `true` (prevents abuse)
+- **WS_FALLBACK**: `false` (disables WebSocket fallback)
+
+**Usage**:
+1. Access https://drop.pivoine.art on both devices
+2. Devices will automatically discover each other if on the same network
+3. For different networks, STUN servers enable peer discovery
+4. Click on discovered device to share files or text
+
+**Technical Details**:
+- Uses WebRTC for direct peer-to-peer connections
+- STUN servers help with NAT traversal and cross-network connections
+- Configuration file mounted at `/rtc_config.json` with Google STUN servers
+- No data persisted - stateless service
+
+**Note**: PairDrop doesn't require backups as no data is stored on the server.
+
 ### Jellyfin (jelly/compose.yaml)
 Jellyfin media server for streaming photos and videos:
 - **jellyfin**: Jellyfin app exposed at `jelly.pivoine.art:8096`