dot versions of folder.jpg do not show up as thumbnails on volume root, unless explicilty specified in th-covers

Hi :)

The docs mention:

> images with the following names (see --th-covers) become the thumbnail of the folder they're in: folder.png, folder.jpg, cover.png, cover.jpg

> if you enable [file indexing](https://github.com/9001/copyparty?tab=readme-ov-file#file-indexing), it will also try those names as dotfiles (.folder.jpg and so), and then fallback on the first picture in the folder (if it has any pictures at all)

I have file indexing enabled (e2dsa+e2ts) without an explicit `--th-covers` list, and indeed, when I put either a `folder.png` or a `.folder.png` into a directory - it shows up as a thumbnail. (Though I think `.folder.png` required me to reload copyparty once?)

With that, If I put `folder.png` into the root of a volume, it also works as expected. But `.folder.png` in the root of a volume does not work "automatically". You can test this by renaming `folder.png` to `.folder.png` back and forth. It works fine in an inner folder, but not in the volume root.

However, it works fine when adding `.folder.png` to the `--th-covers` list explicitly.

### Server details (if you're NOT using docker/podman)
remove the ones that are not relevant:
* **what copyparty did you grab:** sfx.py
* **how you're running it:** systemd-service
* run copyparty with `--version` and grab the last 3 lines (they start with `copyparty`, `CPython`, `sqlite`) and paste them below this line:
```
copyparty v1.19.21 "usernames" (2025-12-02)
CPython v3.13.5 on Linux64 6.12.48 [GCC 14.2.0]
sqlite 3.46.1*1 | jinja 2.11.3 | pyftpd 1.5.10 | tftp 0.4.0
```
config:

```yaml
[global]
name: Gremy Data
shr: /shares
e2dsa
e2ts

[/]
/srv/copyparty/
accs:
r: *
flags:
daw

[accounts]
testman: whatev

[/test]
/srv/copyparty/test
accs:
rwmda.: testman

```