fix(cache): recover offline wallpapers, cache favicons, purge orphane…

[pooooooriya]

…d caches

• Wallpapers (image/gif/video) no longer stay black after an update or while offline. The image is probed out-of-band and re-applied once the service worker has pinned/cached it, videos retry on error, and everything re-applies when the tab comes back online instead of waiting for a manual re-select.
• Bookmark favicons (Google's favicon service) are now cached, so they load offline and hit the network far less. Bounded via ExpirationPlugin and versioned like the other caches.
• Three legacy cache names (images-cache-v1, cross-origin-cache-v1, widgetify-api-cache-v1) from pre-overhaul Workbox versions were missing from the purge list and lingered forever on updated installs; they are now removed by purgeStaleCaches().