refactor: reorganize feat.rs into modular structure (#2827)
Split the monolithic feat.rs file into specialized modules:
- backup.rs: WebDAV backup and restore functions
- clash.rs: Core management and testing functions
- config.rs: Configuration handling
- profile.rs: Profile management
- proxy.rs: Proxy and TUN mode controls
- window.rs: Dashboard window management
This improves code organization, readability, and maintainability
by grouping related functionality into logical modules.
