Studio chat: transcript, composer, and live-session layout #1847

Parent: #1736. Design review: #1779.
Design baseline: 2026-09-24.

Outcome

The existing streamed chat flow uses the agreed transcript and composer layout without losing working controls.

Scope

• Align flat transcript rows, markdown/code/tool rendering, the message status line, responsive composer, attachments, voice input, model and permission-mode selectors, and queue/steer behavior.
• Bring in seed prompts, live session-title adoption, the return-from-away notice, and scheduled-delivery notes where the current event stream provides the needed facts.
• Keep session/run identifiers and SDK types behind the BFF; preserve streamed replay, cancellation, retries, folder state, and account-scoped browser data.

Acceptance

• A user can send, queue, steer, and continue a streamed run from desktop and mobile layouts; Enter during IME composition never sends a prompt.
• Attachments and voice input work under the actual CSP and browser permissions; a blocked capability presents a usable fallback.
• Transcript updates remain usable for long runs; focused stream/composer tests and task studio:check pass, with browser journeys coordinated through Studio test coverage: Playwright journeys, apps/web unit gaps, CI job #1776.

Delivery

Bounded chat acceptance plan; do not broaden daemon authority.

Depends on: #1843, #1844.