feat: replace neko with native GStreamer WebRTC for desktop streaming

Replace the neko binary dependency with a native GStreamer pipeline
(ximagesrc -> vp8enc -> webrtcbin) for desktop video streaming. This
removes the external neko process and integrates screen capture directly
via gstreamer-rs crate bindings behind a `desktop-gstreamer` feature flag.

Key changes:
- Add desktop_gstreamer.rs with GStreamer WebRTC pipeline management
- Rewrite signaling protocol (ready/offer/answer/candidate over WS)
- Add leaky queues and videorate for low-latency streaming
- Rewrite ICE candidates to 127.0.0.1 for Docker connectivity
- Constrain UDP port range (30000-30100) via libnice agent
- Update TypeScript SDK desktop-stream.ts for new signaling
- Update inspector DesktopTab with WebRTC Live View
- Update Dockerfiles to install GStreamer dev packages

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

justfile

@@ -76,6 +76,26 @@ run-gigacode *ARGS:
 dev-docs:
 	cd docs && pnpm dlx mintlify dev --host 0.0.0.0
 
+# Start the desktop dev stack (sandbox-agent backend in Docker + inspector frontend)
+[group('server')]
+server-dev:
+	docker compose -f server/compose.dev.yaml up --build --force-recreate -d
+
+# Stop the desktop dev stack
+[group('server')]
+server-dev-down:
+	docker compose -f server/compose.dev.yaml down
+
+# Tail desktop dev stack logs
+[group('server')]
+server-dev-logs *ARGS:
+	docker compose -f server/compose.dev.yaml logs -f --tail=200 {{ ARGS }}
+
+# Rebuild and restart only the backend container
+[group('server')]
+server-dev-restart-backend:
+	docker compose -f server/compose.dev.yaml up --build --force-recreate -d backend
+
 install:
     pnpm install
     pnpm build --filter @sandbox-agent/inspector...