fix(categories): keep notes when deleting a category

[MiMoHo]

Summary

Deleting a category currently deletes the category folder including all note files inside it — a single confirmation click can destroy many notes. This change makes "Delete category" keep the notes: they are moved to Uncategorized instead, and only the empty folder goes away.

How it works

• All notes of the category (including subcategories) are moved to the notes root via the existing per-note category mechanism (Note::setCategory('')), so file name collisions are resolved the same way as when changing a single note's category (Title (2).md).
• Afterwards, only folders that contain no files at all are removed (new helper deleteFoldersWithoutFiles). Non-note files that happen to live in the category folder are never deleted.
• The confirmation dialog and the client-side store update reflect the new behavior.

Note on the behavior change

This intentionally changes the semantics of an existing action, based on the expectation that "deleting a folder" should not silently destroy its notes. If you would rather keep the destructive delete and offer a choice (e.g. "delete notes" vs. "keep notes"), I am happy to rework this accordingly.

Testing

• Verified end-to-end on Nextcloud 35 (dev): category folder removed, all notes kept, a colliding file name was renamed to Name (2).md, non-note files untouched.
• php -l clean, production webpack build clean.

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🤖 AI (if applicable)

• This pull request was prepared with assistance from Claude Code (Claude Fable 5 xhigh).
  I reviewed and tested the changes myself.