fix: keep custom model state consistent with disk

[ShiroKSH]

Summary

• reject custom model folders without a root Live2D model descriptor before copying or persisting them
• remove stale model records when their backing directory is already missing
• keep the saved model list and current selection intact when filesystem deletion fails

Root cause

The importer treated a successful directory copy as a valid model import, while the loader requires a root *.model3.json file. The delete path updated Pinia state from finally, so it discarded the model record after both successful and failed filesystem operations.

Checks

• eslint src
• tsc --noEmit
• vite build

[ShiroKSH]

@ayangweb We'd appreciate it if you could take a look at this pull request when you have a chance.