[Feature Request]: Improve loading performance for large libraries and folder-based comics

### Preflight Checklist - [x] I have searched the [issue tracker](https://www.github.com/ollm/OpenComic/issues) for a feature request that matches the one I want to file, without success. ### Describe your suggested feature First of all, thank you for building OpenComic. It is currently the best comic reader I’ve found on macOS for my workflow, and I liked the project enough to make a small sponsorship as a token of appreciation. What I especially appreciate is that OpenComic supports folders, subfolders as chapters, CBZ and ZIP files, while still providing a polished and pleasant reading experience. The interaction design feels thoughtful, page turning is generally fast and stable, and despite being Electron-based, it performs better than many established comic readers I’ve tried. That said, there is still one area where the experience could be improved: loading performance. When opening a library or loading a folder-based comic directory, there is a noticeable loading delay before the content becomes available. This is especially visible with larger libraries or comics organized as folders with many chapter subdirectories. It would be great if OpenComic could improve this part of the experience, for example by optimizing directory scanning, caching folder metadata, lazy-loading chapter information, or showing previously cached library content while updating in the background. ### Additional Information _No response_ ### Sample file _No response_