Feature Request: Known Folder Move (Desktop/Documents Redirection)

## Summary
Introduce a "Known Folder Move" feature in Nextcloud Desktop Client, similar to Microsoft OneDrive.
This would allow users to seamlessly redirect system folders (e.g., Desktop, Documents, Pictures) into the Nextcloud sync directory, ensuring transparent backup and sync without manual configuration.

### Platforms
- [ ] macOS
- [ ] Windows

## Motivation
- Many users expect cloud sync clients to automatically protect their most important folders.
- OneDrive offers a wizard that moves contents and creates links so applications continue to work without disruption.
- Nextcloud currently requires manual moves or symlink/junction hacks, which are error‑prone and inconsistent across platforms.
- Implementing this natively would improve usability, adoption, and reduce support overhead.

## Proposed Design
1. **Folder Selection UI**
- Add a wizard in the Nextcloud Desktop Client to select known folders (Desktop, Documents, Pictures).
- Detect OS and present appropriate defaults.

2. **Migration Logic**
- Move contents from original folder into Nextcloud sync root.
- Create symlink/junction at original location (platform‑specific: `ln -s` on Linux/macOS, `mklink /J` on Windows).
- Track migration state to avoid duplicate moves.

3. **Continuous Sync**
- Once redirected, the Nextcloud client syncs the new location automatically.
- Provide status indicators in the client UI.

4. **Safety & Rollback**
- Error handling for permissions, partial moves, or failed symlink creation.
- Option to roll back and restore original folder layout.

## Benefits
- Transparent backup of critical user folders.
- Cross‑platform consistency.
- Reduced need for manual symlink/junction setup.
- Improved user experience and parity with commercial cloud solutions.

## Alternatives
- Manual symlink/junction creation (current workaround).
- Multi‑folder sync configuration (requires advanced user knowledge).

## Next Steps
- Discuss feasibility with maintainers.
- Define cross‑platform implementation details.
- Prototype migration logic in the desktop client.

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### Checklist
- [ ] Feature request aligns with Nextcloud’s mission of user‑controlled cloud sync.
- [ ] Cross‑platform considerations documented.
- [ ] Usability and rollback safety addressed.

No files, tests, or entry points are named. Start by reviewing the existing multi-folder sync configuration and the platform-specific folder handling, then define feasibility for macOS and Windows, migration and rollback behavior, and the UI flow; done requires an agreed implementation plan before prototyping.

Written by the indexing model from the issue text.