Settings: allow removing a secondary library path mapping

[Brandon-Haney]

Summary

Adds a way to remove a secondary library path mapping from Settings > Libraries. Previously a library could accumulate extra path mappings (for example after auto-fill, or when a library spans multiple roots) with no way to delete one from the UI; the only options were edit or leave it. This adds a per-mapping Remove action.

What changed

• UI: Each mapping in a library card's edit view gets a Remove button, shown only when the library has more than one mapping (so you can't delete a library's last mapping and orphan it). Clicking it opens a styled confirmation modal naming the mapping, consistent with the rest of the Settings dialogs, rather than a raw browser confirm.
• Backend: update_library_paths honors a delete_<pos> flag per mapping. Flagged mappings are dropped from path_mappings instead of updated, then valid_sections is rebuilt and settings saved. Deletions are applied highest-index-first so earlier indices stay valid.

How to test

1. Open Settings > Libraries and pick a library that has two or more path mappings (add a second one first if needed).
2. Edit the library — each mapping now shows a Remove button.
3. Click Remove on a secondary mapping, confirm in the modal, and save.
4. The mapping is gone from the card and from plexcache_settings.json (path_mappings), and the library still works with its remaining mapping(s).
5. Confirm a library with only one mapping shows no Remove button (can't be left with zero).

Covered by tests/test_library_paths_delete.py.