Add automated test coverage

[evanjt]

Expands automated test coverage with no production-code changes. Tests use recorded fixtures, not the live API; distances are deterministic from the pure geo functions.

Android:

• Logic — deterministic geo distances (Planta→Sion), Departure model, line-pill prefixes, JSON round-trips.
• Widget logic — WidgetFavourites.block, WidgetFetchResult selection, refresh window.
• Snapshots (Roborazzi, light + dark) — departure rows, tracking bar + formation, Paused screen.

iOS and Garmin follow on this branch.