fix(ui): surface websocket send failures

[LiuShenXi]

Summary

• Make the shared WebSocket sendMessage helper report whether a frame was actually sent.
• Propagate that result through startSessionCommand so the chat composer can detect failed sends.
• When a chat command cannot be sent because the WebSocket is not connected, keep the draft input, avoid entering the optimistic Processing state, and show a visible error message instead of only logging to the console.
• Add regression coverage for successful/failed send reporting and the visible failure message.

Test Plan

• pnpm --filter pilotdeck-ui test
• pnpm --filter pilotdeck-ui build

Notes

• pnpm --filter pilotdeck-ui typecheck currently fails on the base branch with existing React type-version conflicts and an existing handleCommandInputChange signature error; no new sendMessage/startSessionCommand type errors were found when filtering the output for touched APIs.