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/architecture.md

@@ -36,18 +36,35 @@ The repository now includes separate runnable workspaces for the UI and gateway
 - PWA enabled
 - WebSocket client
 
-The current implementation is a minimal SvelteKit app with a single starter page. PWA behavior, microphone capture, and the WebSocket client are later increments.
+The current implementation is a minimal SvelteKit app with a single voice-session shell page. The shipped UI can open and close a browser WebSocket connection to the gateway `/ws` endpoint, show explicit connection status (`not connected`, `connecting`, `connected`, `disconnected`, `error`), and surface session metadata for developers. Microphone capture, transcript rendering, interrupt controls, streamed assistant response display, and audio playback are not part of the current shell and remain future work.
 
 #### Responsibilities
 
+Current shell responsibilities:
+
+- connection state rendering
+- developer-oriented session metadata rendering
+- browser session connect/disconnect controls
+
+Future UI responsibilities:
+
 - audio capture from microphone
 - audio playback for TTS
-- UI state rendering
-- session management
+- broader voice-session UI state rendering
 - interrupt handling
 
 #### Main Screen
 
+Current shell:
+
+- developer-focused voice-session panel
+- connect button
+- disconnect button
+- connection status indicator
+- session metadata display
+
+Future interactive voice screen:
+
 - large mic button
 - live transcript
 - streamed assistant response text
@@ -85,6 +102,14 @@ The current implementation is a minimal Fastify service with `/`, `/health`, and
 - valid minimal client events can move the session between `idle` and `listening`
 - invalid JSON, invalid envelopes, and malformed frames are handled defensively so the process stays up
 
+### Current UI shell behavior
+
+- renders a minimal developer-focused voice-session panel
+- exposes connect and disconnect controls only
+- does not request microphone permission
+- does not send or process audio data
+- reads `session.ready`, `session.state`, and `error` messages from the shared protocol contract
+
 ## Voice Pipeline
 
 ```text