Offline: automatic updates, persistent storage and quota handling

Apple keeps downloaded maps up to date automatically and warns when storage is low. OfflineMaps supports manual update(id), and nothing guards the browser storage quota.

Scope

• Automatic updates: an opt-in autoUpdate with a maximum age. On ready(), when online (and ideally on unmetered connections, where navigator.connection says), refresh regions older than it, one at a time. Add a switch in the Offline Maps panel, as Apple has.
• Persistence: call navigator.storage.persist() when the first region is downloaded, so the browser does not evict downloads under pressure. Show whether storage is persisted.
• Quota: check navigator.storage.estimate() before a download. Warn in the area picker when the estimate exceeds the free space, and fail gracefully (status error, a clear message) on QuotaExceededError.
• Automatic resume: an interrupted download currently comes back paused after a reload. Add an option to resume automatically when back online.