Config: Add an option to disable the web user interface

[lastzero]

As an admin, I would like to disable the web frontend for instances that should only handle API requests.

For this purpose, we will add the following new feature flag to the config options:

Environment	CLI Flag	Default	Description
PHOTOPRISM_DISABLE_FRONTEND	--disable-frontend		disable the web user interface so that only the service API endpoints are accessible

[lastzero]

An updated preview build will be available soon to test these changes:

• https://docs.photoprism.app/getting-started/updates/#development-preview