Implement web requests on Rust backend to avoid CORS

- Add web module in src-tauri/src/extension/web/mod.rs
- Implement extension_web_fetch Tauri command using reqwest
- Add WebError variant to ExtensionError enum
- Update frontend handler to call Rust backend via Tauri IPC
- Web requests now run in native context without CORS restrictions

src-tauri/src/lib.rs

@@ -78,6 +78,7 @@ pub fn run() {
             database::vault_exists,
             extension::database::extension_sql_execute,
             extension::database::extension_sql_select,
+            extension::web::extension_web_fetch,
             extension::get_all_dev_extensions,
             extension::get_all_extensions,
             extension::get_extension_info,