Library persistence fails if user's Templates folder was deleted

Whiteboard library persistence fails if the user's "Templates" folder has been deleted.

Environment

• Nextcloud deployment: Nextcloud AIO
• Whiteboard version: 1.5.9
• Browser: Brave / Chromium

Steps to reproduce

1. Remove the user's Templates/ folder from Nextcloud Files (or rename to TemplatesBackup)
2. Open any .whiteboard file.
3. Import a library. Library -> three dots -> Open -> select previously downloaded .excalidrawlib from a file picker
4. Confirm the imported library appears in the current session.
5. Reload the page and enter the .whiteboard file.
6. Imported library disapears.

Actual behavior

The imported library disappears after reload.

In browser DevTools, the library save request fails:

PUT /apps/whiteboard/library -> 404

Expected behavior

Whiteboard should persist the imported library, even if the user deleted the default Templates/ folder.

Possible solution

1. Ensure the user's Templates/ folder exists before reading/writing personal.excalidrawlib.
2. Recreate Templates/ folder when importing a library.

Reproduce the deleted-Templates scenario and inspect the failing PUT /apps/whiteboard/library request, including how the personal.excalidrawlib location is handled. Done means an imported library remains available after reload even when the user's Templates folder was deleted.