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.

src-tauri/src/feat/mod.rs

@@ -0,0 +1,14 @@
+mod backup;
+mod clash;
+mod config;
+mod profile;
+mod proxy;
+mod window;
+
+// Re-export all functions from modules
+pub use backup::*;
+pub use clash::*;
+pub use config::*;
+pub use profile::*;
+pub use proxy::*;
+pub use window::*;