Docs: add canvas host usage

docs/ios/spec.md

@@ -30,6 +30,7 @@ Non-goals (v1):
 
 ## Current repo reality (constraints we respect)
 - The Gateway WebSocket server binds to `127.0.0.1:18789` (`src/gateway/server.ts`) with an optional `CLAWDIS_GATEWAY_TOKEN`.
+- The Gateway can optionally expose a LAN/tailnet Canvas file server (`canvasHost`) so nodes can `canvas.navigate` to `http://<lanHost>:<canvasPort>/` and auto-reload when files change (`docs/configuration.md`).
 - macOS “Canvas” exists today, but is **mac-only** and controlled via mac app IPC (`clawdis-mac canvas ...`) rather than the Gateway protocol (`docs/mac/canvas.md`).
 - Voice wake forwards via `GatewayChannel` to Gateway `agent` (mac app: `VoiceWakeForwarder` → `GatewayConnection.sendAgent`).