Config: Use dynamic social preview image based on app name

[lastzero]

As a user sharing links to my own instance, I want the preview image visible on social media and in messengers to reflect the app name I have chosen in the configuration.

This only affects the default setting. Sponsors can continue to set a custom image using the PHOTOPRISM_SITE_PREVIEW variable:

• https://docs.photoprism.app/getting-started/config-options/#site-information