feat(vela-ui): add voice session shell

Add a minimal UI shell that connects to the gateway WebSocket and exposes developer-visible session state. Align the architecture, protocol, setup, integration, and backlog docs with the current UI increment.

docs/integrations.md

@@ -5,6 +5,7 @@
 - `vela-ui` is implemented as a SvelteKit application
 - `vela-gateway` is implemented as a Fastify service
 - `vela-gateway` now exposes `/ws` as the minimal WebSocket session entrypoint using the shared `@vela/protocol` contract
+- `vela-ui` now opens a minimal browser WebSocket client against that `/ws` entrypoint and surfaces connection/session status for developers
 - current integration work beyond the gateway WebSocket/session baseline remains future implementation
 
 ## Gateway Session Contract
@@ -14,6 +15,7 @@
 - message format: `@vela/protocol` `MessageEnvelope<{ type, payload }>`
 - current server behavior: acknowledge connect with `session.ready` and `session.state`
 - safety baseline: invalid JSON, invalid envelopes, and malformed frames return protocol errors or close that socket without taking down the service
+- current UI behavior: connect/disconnect only, no microphone access, no audio payloads, and safe error-state handling for `open`/`error`/`close`
 
 ## STT (Speech-to-Text)