fix(tui): apply model selection on blank submit

[thdxr]

Issue for this PR

User-reported; no linked issue.

Type of change

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

What does this PR do?

Blank Enter now commits the composer selection before the queued-prompt action, then returns without sending an empty message or clearing the composer.

The submission flow shares selection preparation and failure/restoration handling, consolidates queue checks, and handles blank submission explicitly. Revert still finishes before optimistic prompt admission; model commits remain ordered after earlier admissions.

How did you verify your code works?

• bun run check passed.
• 78 existing tests passed, including real Prompt command-selection and compaction-ordering tests.
• Fable used temporary full-app tests to compare eight scenarios against both the pre-refactor and refactored implementations: model/agent switching and queue promotion, unchanged selections, switch failure, blank queue/shell submissions, revert ordering, queued exit rejection, and home-session creation. All passed; temporary tests were removed.

Screenshots / recordings

None; submission behavior change with no layout changes.

Checklist

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

from OpenCode