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>

docker/runtime/Dockerfile

@@ -153,7 +153,15 @@ RUN apt-get update && apt-get install -y \
     ca-certificates \
     curl \
     git \
-    ffmpeg && \
+    ffmpeg \
+    gstreamer1.0-tools \
+    gstreamer1.0-plugins-base \
+    gstreamer1.0-plugins-good \
+    gstreamer1.0-plugins-bad \
+    gstreamer1.0-plugins-ugly \
+    gstreamer1.0-nice \
+    gstreamer1.0-x \
+    libgstreamer1.0-0 && \
     rm -rf /var/lib/apt/lists/*
 
 # Copy the binary from builder