Expose STT tunables in UI settings

[acgxv]

Context: All tunables should be adjustable from the UI; env vars remain dev-only (Status: TODO).

Scope:

• Enumerate the tunables that should be exposed.
• Add UI controls for those tunables.
• Keep env var overrides as dev-only.

Reference: docs/spec/core/stt_dictation_pipeline.md (UI-first Settings).

Acceptance:

• UI settings exist for all required tunables.
• Env vars are optional and dev-only.
• Spec updated to list the tunables and defaults.