Better Folder Thumbnails™

**Is your feature request related to a problem? Please describe.**
For media libraries, the most representative file is often the largest file in the folder (e.g. the main movie, the highest-resolution photo). Manually creating symlinks or renaming files to match --th-covers is cumbersome and clutters directories.

**Describe the idea / solution you'd like**
Add a new option that tells copyparty to use the largest file (by byte size) inside a folder as its thumbnail, provided that file is of a type that copyparty can generate a thumbnail from (image, video, or any format handled by the thumbnail pipeline).

--folder-thumb-largest would be ideal.

**Describe any alternatives you've considered**

- Using --th-extract - but this only works for files with extensions, not for folders; there is no hook for folder thumbnail generation.
- A cron script that creates folder.jpg symlinks - works but writes extra files into every directory and needs periodic maintenance.
- Accepting the current default – often picks an arbitrary small image or an image from the wrong video in a folder, not the main one.

查看代码库中的缩略图生成逻辑，可能位于处理缩略图或媒体索引的模块中。找到当前选择文件夹缩略图的位置，并修改为按文件大小选择最大的文件。通过设置一个包含混合媒体文件的文件夹进行测试，并验证缩略图会随新选项发生变化。

由索引模型根据 Issue 内容生成。