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>

server/compose.dev.yaml

@@ -0,0 +1,40 @@
+name: sandbox-agent-dev
+
+services:
+  backend:
+    build:
+      context: ..
+      dockerfile: docker/test-agent/Dockerfile
+    image: sandbox-agent-dev
+    command: ["server", "--host", "0.0.0.0", "--port", "3000", "--no-token"]
+    environment:
+      RUST_LOG: "${RUST_LOG:-info}"
+    ports:
+      - "2468:3000"
+
+  frontend:
+    build:
+      context: ..
+      dockerfile: server/docker/frontend.dev.Dockerfile
+    working_dir: /app
+    depends_on:
+      - backend
+    environment:
+      SANDBOX_AGENT_URL: "http://backend:3000"
+    ports:
+      - "5173:5173"
+    volumes:
+      - "..:/app"
+      # Keep Linux-native node_modules inside the container.
+      - "sa_root_node_modules:/app/node_modules"
+      - "sa_inspector_node_modules:/app/frontend/packages/inspector/node_modules"
+      - "sa_react_node_modules:/app/sdks/react/node_modules"
+      - "sa_typescript_node_modules:/app/sdks/typescript/node_modules"
+      - "sa_pnpm_store:/root/.local/share/pnpm/store"
+
+volumes:
+  sa_root_node_modules: {}
+  sa_inspector_node_modules: {}
+  sa_react_node_modules: {}
+  sa_typescript_node_modules: {}
+  sa_pnpm_store: {}