[错误]: 单专辑艺术家的图像分辨率失败 ("无法找到艺术家的文件夹")

I confirm that:

• I have searched the existing open AND closed issues to see if an issue already exists for the bug I've encountered
• I'm using the latest version (your issue may have been fixed already)

Version

0.63.2

Current Behavior

For an artist with exactly one album in the library, Navidrome fails to resolve any artist image — not the local artist folder image, and not a fallback external image either. The artist page just shows the default placeholder, even for well-known artists (Coldplay, in this case) that should resolve easily via Deezer/Last.fm/ListenBrainz. Album cover art for the same album works correctly and displays fine — only artist-level image resolution is affected. The logs show Navidrome computing an artist folder path that looks correct as a string, but failing to find a matching folder entry for it:

time="2026-07-19T18:03:29Z" level=warning msg="Parent folder not found for album cover art lookup" parentID=5wS0qoLbB6VxE2B1NJDwyF requestId=69df5158-7bba-4d20-a3fd-7853cac37740
time="2026-07-19T18:03:29Z" level=warning msg="Could not find folder for artist" folderPath=/music/media/music/Coldplay id=5wS0qoLbB6VxE2B1NJDwyF libID=1 libPath=/music/media/music requestId=69df5158-7bba-4d20-a3fd-7853cac37740

This matches an explanation given previously in the developer's comment (https://GitHub.com/navidrome/navidrome/discussions/3676#discussioncomment-12007173): the artist folder is inferred by computing the longest common prefix across all of an artist's album folder paths, which breaks down when there is only one album to compare against. Additionally, this doesn't appear to gracefully fall through to the external step of ArtistArtPriority once the local folder resolution fails — instead of getting an external image (e.g. from Deezer), the placeholder is shown.

Expected Behavior

For single-album artists, artist image resolution should either:

1. Correctly resolve the artist folder even with only one album (e.g. by using the album's parent directory directly when there's exactly one album, rather than relying on a multi-path common-prefix calculation), or
2. Gracefully fall through to the next ArtistArtPriority source (album/artist.*, then external) when local folder resolution fails, instead of stopping at the placeholder.

Steps To Reproduce

1. Add an artist to the library that has exactly one album (no other albums/tracks anywhere else in the library for that artist).
2. Open the artist's page in the Navidrome web UI.
3. Observe the placeholder (grey star) image is shown instead of any local or externally-fetched artist image.
4. Check logs for "Could not find folder for artist" warnings tied to that artist's ID.

Environment

- OS: Kubernetes (containerized)
- Browser: N/A (also reproduces via Subsonic/OpenSubsonic clients)
- Client: Navidrome WebUI
### How Navidrome is installed?