Add delete confirmation dialogs

### Problem

When users delete files in a synced folder, there's no warning. This causes accidental data loss, especially for users unfamiliar with sync behavior.

OneDrive and Dropbox both have safety dialogs that Nextcloud lacks:

- "First delete" educational popup explaining files will be deleted everywhere
- Confirmation when deleting shared files or moving files out of sync folder

### Proposed Solution

- First-time delete dialog: Explain that deletion syncs everywhere + mention trash recovery (with "Don't show again" checkbox)
- Shared files confirmation: "This file is shared with X people. Delete anyway?"
- Mass deletion warning: "You're about to delete X files. Continue?" (configurable threshold)