Feature: Persist/Restore Theme (dark/light/system)

### Why
Actually use the persistence framework to store and restore UI theme preference (dark/light/system), defaulting to system if not set. Support changing via UI, and restoring reliably across reloads.

### Acceptance Criteria
- Theme setting is written to localStorage using new service
- Theme setting is read on startup and applied early (before main render)
- UI exposes setting and mutation (toggle/button)
- Tests: setting persists across tab/browser sessions
- Initial value can honor browser/OS `prefers-color-scheme` if no value saved