Offline: keep downloading in the background and resume automatically

Offline maps are downloaded in the foreground. If the tab is closed or the device sleeps, the download stops. It comes back paused on the next visit, and the user has to press Resume.

Apple downloads in the background and resumes by itself.

Scope

• Resume paused regions automatically when the app is next opened and online (opt-in; see also #6255).
• Explore Background Fetch (BackgroundFetchManager) or a service-worker-driven download, so large regions finish with the tab closed, where supported.
• Report progress in the Offline Maps panel across reloads.