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/rtc_config.json

@@ -0,0 +1,20 @@
+{
+  "sdpSemantics": "unified-plan",
+  "iceServers": [
+    {
+      "urls": "stun:stun.l.google.com:19302"
+    },
+    {
+      "urls": "stun:stun1.l.google.com:19302"
+    },
+    {
+      "urls": "stun:stun2.l.google.com:19302"
+    },
+    {
+      "urls": "stun:stun3.l.google.com:19302"
+    },
+    {
+      "urls": "stun:stun4.l.google.com:19302"
+    }
+  ]
+}