Add offline caching layer to Stats Repository

Use the Decorator pattern: `CachedStatsRepository implements StatsRepository` and takes an inner `StatsRepository` as a constructor parameter. Register via Riverpod as `cachedStatsRepositoryProvider` that overrides `statsRepositoryProvider`. Stale-while-revalidate is best implemented with a `StreamController.broadcast()` per cache key — the repository emits the cached value synchronously, then emits again after the network response if values differ. Use `jsonEncode`/`jsonDecode` for serialisation rather than Hive TypeAdapters to avoid code-gen complexity for this cache layer.

Cache key hashing: sort `chapterIds` alphabetically before hashing to ensure key stability regardless of list order. Eviction on startup: iterate all keys in the Hive box and delete entries where `cachedAt` is older than 1 hour — this prevents unbounded disk usage over time. The `connectivity_plus` package provides a stream of `ConnectivityResult`; check it before attempting network fetch and set `CacheSource.stale` on the returned result wrapper.

Unit tests using `mocktail`: (1) cache miss — delegates to inner repository and stores result; (2) cache hit within TTL — returns cached data and fires background fetch; (3) cache hit beyond TTL — treats as miss; (4) offline state — returns stale cache with `CacheSource.stale` flag; (5) clearCache() empties all stats entries; (6) background re-fetch emits update when data changes. Use a fake `HiveInterface` or in-memory map for Hive in unit tests. Integration test: cold-start app, fetch data, disable network, re-open stats screen — verify cached data is shown within 50 ms.