Improve functionality of File System Access API, especially persistent permissions and refresh

[Jaifroid]

Unlike with the PWA (https://pwa.kiwix.org) the browser extension appears to lose access to persistent FSA permissions between app launches, which defeats the purpose of using this API. This appears to be a regression, or else it was never properly implemented in the first place.

Additionally, the app lacks "refresh" capabilities for picked folders: on first launch it should retrieve the latest file list and update if necessary. There should also be a refresh button for when the user has manually added a new ZIM archive, instead of having to re-pick the folder.