refactor(app): remove promoting workaround

[nokados]

Issue for this PR

N/A — cleanup of workaround from #34466

Type of change

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

What does this PR do?

Commit e34822d (#34466) added an in-memory promoting state to
bridge the gap between promote() clearing store.draft and
navigate() updating the session ID.

Commit ea982c3 (#36182) wrapped promote() + navigate() in
startTransition, which batches both updates atomically. The
promoting workaround is no longer needed — scope() switches
directly from store.draft to saved.session[session] with no
intermediate state.

How did you verify your code works?

Verified no flicker on both latest and dev Web UI channels when
sending the first message in a new session with a non-default model
selected.

Screenshots / recordings

Checklist

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

[github-actions]

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