Define settings UX for dictation method configuration

[acgxv]

Context: Dictation method configuration (streaming-only, local two-pass, local one-shot) is deferred.

Decisions to make:

• Where and how to present the mode selection in settings.
• Copy and descriptions to set expectations on latency and quality.
• Any constraints or warnings for incompatible modes.

Reference: docs/spec/ui/app_experience.md

Acceptance:

• Add the finalized settings UI and behavior to the spec.