refactor: src-tauri 重命名为 gui

Tauri CLI 自动检测 workspace 成员，构建不受影响。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

gui/src/commands/backup.rs

@@ -0,0 +1,6 @@
+use path_editor_core::backup;
+
+#[tauri::command]
+pub fn backup_registry(custom_dir: Option<String>) -> Result<String, String> { backup::backup_registry(custom_dir) }
+#[tauri::command]
+pub fn get_appdata_dir() -> String { backup::get_appdata_dir() }