Function fetch_with_cache 

pub async fn fetch_with_cache<T>(
    offline: bool,
    load_cache: impl FnOnce() -> Result<Option<CachedData<T>>>,
    save_cache: impl FnOnce(&T) -> Result<()>,
    fetch_api: impl Future<Output = Result<T>>,
) -> Result<Option<T>>

Fetch data with cache fallback.

• offline — if true, return cached data only (regardless of staleness)
• load_cache — loads cached data (returns Ok(None) if absent)
• save_cache — saves fetched data to cache
• fetch_api — async API fetch

Returns:

• Offline + cached → Ok(Some(data))
• Offline + no cache → Ok(None)
• Cache fresh → Ok(Some(data))
• API success → saves + Ok(Some(data))
• API error + stale cache → Ok(Some(stale_data))
• API error + no cache → Err