Automatic categorization for batch downloads, plus player and feed ads in the built-in CMS

We recently added a few features, mainly for the built-in CMS, and fixed several issues:

1. Added category selection to batch downloads in the download center. Choose a category when adding downloads; completed downloads are categorized automatically, and nonexistent categories are created automatically.
2. Added webp uploads for galleries. The CMS gallery detail page now displays one image per page and preloads the next image while the current one is viewed.
3. Added announcement management to the top of the CMS homepage. Multiple announcement titles rotate in turn. Clicking a title displays its content, which supports Markdown syntax.
4. Removed standalone image ads from the CMS and added video feed ads, which appear like videos among content on all pages. When multiple feed ads are configured, one or more are selected randomly for display.
5. Added player ads to the CMS, in 2 types: self-managed and advertiser-provided. Self-managed ads currently support pre-roll only. Advertiser-provided player ads must follow the VAST standard, with display rules determined by the advertiser. When multiple ads of either type are configured, one is selected randomly for playback.
6. Added a P2P switch for the CMS player and configuration for free P2P nodes. You need to deploy your own nodes; the documentation includes a quick setup guide.
7. Changed cms resolution selection. Ordinary users and guests play at the lowest resolution by default. Members' selected resolution is remembered and used automatically for subsequent videos.
8. Added preview videos to all CMS pages. Hovering over a cover for more than 0.5s automatically plays the preview generated during transcoding.
9. Fixed verification failures for m3u8 playlists containing a key when preview mode and timestamp-based hotlink protection are both enabled. Also fixed the lack of a notification when cloud-storage synchronization is disabled.

Related screenshots

For more frontend details, see the demo site: https://www.leimulamu.com/