Design history and metrics surfaces

[acgxv]

Context: History and metrics are required but currently deferred.

Decisions to make:

• Navigation structure (unified dashboard vs separate views).
• History list content (text, timestamps, target app, audio snippet playback).
• Metrics to show (token usage, usage time, ASR character/word count).

Reference: docs/spec/ui/app_experience.md

Acceptance:

• Update the spec with the chosen structure and surface layout.