fix(app): show a steer as pending from its first frame - #55251

A steer sent while the agent works showed as delivered (blue) for a few frames, then faded to pending (gray).

Causes (both needed for the flash)

• Late admission: sendPrompt awaited the agent and model switch before data.session.prompt admitted the optimistic row. Until then the timeline had only the handoff message. It has no pending status, so it rendered as delivered, and as the active turn with Working under it.
• Fade into pending: user-message-text transitions background-color and color over 200 ms. The row computes its style as delivered once before admission marks it pending, so it faded from blue even when admission landed in the same task.

Fix

• composer/submit.ts: pass the selection commit as the data layer's prepare, like the TUI and /compact. The prompt admits locally in the same task as the handoff. The switch still lands before the POST.
  • A staged revert still settles before admission, because its echo deletes local rows from the boundary on. Only an idle session stages a revert, so this path never shows a pending steer.
  • A retried admission reuses the first commit, so a retry does not switch the agent again.
• message-part.css: transition: none in the pending state. Only delivery (gray → blue) animates.

const admission = {
  // ...
  prepare: staged ? undefined : () => (committed ??= commit()),
}

await session.data.session.prompt(admission).catch(() => session.data.session.prompt(admission))

sequenceDiagram
  participant C as Composer
  participant T as Timeline
  participant D as Data layer
  participant S as Server
  Note over C,S: Before
  C->>T: handoff (no pending status: blue)
  C->>S: switch agent / model
  S-->>C: ok
  C->>D: prompt() admits the optimistic row (pending: fades to gray)
  D->>S: POST prompt
  Note over C,S: After
  C->>T: handoff
  C->>D: prompt({ prepare }) admits in the same task (pending: gray)
  D->>S: switch agent / model (prepare)
  D->>S: POST prompt

Model switch held in flight

Before	After

Bubble background on each painted frame after Enter (Chromium, local mock server)

Model switch slowed to 900 ms

Before

After