- add `app_state` module with `ClockAnchor` and `AppState`
- initialize `AppState` with a default `CountdownService`
- register app state in Tauri via `.manage(app_state)`
- extend `CountdownSnapshotDto` epoch fields to `Option<u128>`
- add countdown model helpers for start/target epoch-ms conversion
- return optional epoch fields from `CountdownService::snapshot`
- ignore generated `dist/` directory in `.gitignore`
