feat(desktop): reserve gray message styling for pending steers

[usrnk1]

Issue for this PR

No linked issue; fixes the gray pending-state flash reported in desktop.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

• Keeps normal sends in their normal message style through optimistic admission and server acknowledgement. Gray remains reserved for pending steers.
• Checks whether execution is running at admission time, so rapid follow-ups stay pending even when the first send is still preparing.
• Restores pending styling and queue/delete actions when startup fails or is interrupted before the prompt is delivered.

How did you verify your code works?

• bun run check and bun run lint:changed passed.
• 56 focused composer/client tests passed; the two review regressions failed before their fixes.
• All 27 queue regression tests passed against a production build. Also exercised idle sends, pending steers, and failed startup in the real UI.
• Compared the production cold-session entry benchmark before/after: stable rendering at 256.6 ms / 270.5 ms in single diagnostic runs, with no wrong-session samples.

Screenshots / recordings

Normal send while idle

Before	After

Pending steer	Undelivered prompt after failed startup

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[Brendonovich]

Replaced by #53880