Floccus creates additional lockfile on every sync and doesn't clean them up

### Which version of floccus are you using?

v5.10.2

### How many bookmarks do you have, roughly?

600

### Are you using other means to sync bookmarks in parallel to floccus?

No

### Sync method

WebDAV

### Which browser are you using? In case you are using the phone App, specify the Android or iOS version and device please.

Firefox 154.0 and Android 17 on Pixel 8 with GrapheneOS

### Which version of Nextcloud Bookmarks are you using? (if relevant)

_No response_

### Which version of Nextcloud? (if relevant)

_No response_

### What kind of WebDAV server are you using? (if relevant)

Drive functionality offered by mailbox.org (which is based on Open-Xchange I believe)

### Describe the Bug

Floccus creates a new lock file on every sync and doesn't delete it after the sync completes successfully. Over the time these files accumulate. The naming scheme is `bookmarks.xbel (n).lock` where `n` increments on each sync.

This behavior exists for more than a year, I just happen to notice it now (I had thousands of lockfile remnants in my webdav storage). Also this issue exists in bot the android app and the firefox desktop browser extension

### Expected Behavior

Only a single lockfile shall be created which is also supposed to disappear after the sync completed successfully

### To Reproduce

- Use mailbox webdav storage
- sync

[floccus-5.10.2-2026-08-22-redacted.log](https://github.com/user-attachments/files/31339591/floccus-5.10.2-2026-08-22-redacted.log)

### Debug log provided

- [x] I have provided a debug log file

No source file or test is named in the issue. Start by reproducing a sync against mailbox.org WebDAV and examining the provided debug log, then trace the WebDAV sync lock handling; done means repeated successful syncs leave only the temporary lock during sync and remove it afterward on Firefox and Android.

Written by the indexing model from the issue text.