Define rewrite presentation surface and interaction model

[acgxv]

Context: The spec notes that ASR output must be rewritten before insertion, but the UI surface is undecided.

Decisions to make:

• Where rewritten text appears before insertion (panel replace, dual view, separate review).
• Interaction model (auto-insert vs confirm, cancel/retry, fallback to original).
• Visual feedback for rewrite completion and errors.

Reference: docs/spec/ui/app_experience.md

Acceptance:

• Update the spec with the chosen surface and interaction model.
• Identify any required UI states or backend signals.