Performance issues with initial sync and UI responsiveness

**Which Nextcloud Version are you currently using:** 32.0.3 with Desktop Client 4.0.4 and with Windows 11.

When syncing a large number of files (>100k), several performance problems occur:

**1. Initial sync too slow**
- Problem: First-time sync with very large file counts takes an extremely long time.
- Suggested solutions:
- Implement lazy sync — only sync folders when accessed.
- Cache or save a file database of folders to reduce the overhead of counting and reading each file.
・Users connecting for the first time could simply download the saved database instead of recalculating everything.

**2. UI becomes laggy during sync**
- Problem: The Nextcloud UI is noticeably unresponsive while syncing files.
- Suggested solution: Move sync operations to a separate thread so the UI remains smooth.

**3. File counting takes too long**
- Problem: Counting the number of files to sync consumes excessive time.
- Suggested solution: Optimize or defer file counting to improve responsiveness.