Add configurable port via STORYKIT_PORT env var (default 3001). Server
prints machine-readable STORYKIT_PORT=<port> on startup and writes
.story_kit_port file for discovery. Frontend proxy and WebSocket read
VITE_STORYKIT_PORT env var instead of hardcoding port 3001.
7 new tests (4 backend, 3 frontend) all passing.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>