Add an automatic download option

### Is your feature request related to a problem? Please describe.
No problem now, only a few improvements are needed.

### Describe the solution you'd like
Add an automatic download option. When viewing pictures / videos or other files, they are automatically downloaded locally to avoid downloading each time, saving network traffic and improving the experience. In order to avoid occupying the local space, configure the maximum quota of local space. When the maximum value is encountered, the earliest one with the "last viewing time" is automatically deleted, but only for automatic downloads, and not for manual downloads or locally edited unuploaded ones.

### Describe alternatives you've considered
N/A.

### Additional context
N/A.