feat(chat): queue follow-up messages during responses

[AnthonyRonning]

Summary

• keep the Maple Chat composer available while an assistant response is streaming and stage follow-up turns in a client-owned FIFO queue
• reuse Agent Mode's queued composer card, edit, discard, remove, and draft-restoration behavior without exposing steer/send-now controls
• retain queued text, documents, images, model selection, and web-search selection across conversation navigation and draft-to-conversation rekeying
• preserve queued work on Stop or failure and let an empty Send resume the retained FIFO
• fence automatic promotion while chat history, projects, account state, or credentials are changing, and bind SDK requests to the account that initiated them

Backend dependency

Depends on OpenSecretCloud/opensecret#343. The backend PR must land before this client ships because successful cancellation and terminal response events are used as quiescence barriers.

There is intentionally no server-side queued-message state and no force-push/steer behavior in Chat mode. The queue lives in the current account's client runtime.

Validation

• frontend formatting, lint, TypeScript, production build, and 914/914 tests
• reproducible PR web build
• TypeScript SDK dependency audit, format/build, and credential-free gate: 148 passed, 3 intentional hosted-attestation skips, 0 failed
• Chrome GUI smoke with a managed local Pro account, Kimi K3, live web search, FIFO promotion, edit/remove, Stop/resume, navigation retention, and a 390 x 844 compact viewport
• browser console: no errors during the smoke run

Not verified

• Tauri/native desktop presentation
• deployed enclave, provider, billing, or production rollout behavior

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