fix(server): only allow absolute import paths

[etnoy]

I've seen more than one occasion of users specifying relative paths in their library import paths, one example is #13570.

Apparently, the path validator accepts a relative path as input while it fails when we do the actual scan. This PR ensures that relative paths fail validation as well as give a helpful message to the user which path they possibly are referring to

This is possibly a breaking change since users that currently use relative paths will fail validation

[alextran1502]

What is the behavior of this breaking change? If a user used a a relative path before, what action must they take to have the library working correctly?

[etnoy]

What is the behavior of this breaking change? If a user used a a relative path before, what action must they take to have the library working correctly?

I didn't mark this as breaking because a user who used a relative path as import path will already be experiencing the issues mentioned in #13570.

Before this PR, they would be none the wiser as to why assets are trashed.

After this PR, they will get help: in the library page they will see that the path is failing validation. When they hover over the checkbox, they will learn they should use an absolute path, and Immich will even tell them which path to put in there to fix it.

I therefore think we don't need to put this as a breaking change.