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/registry.rs

@@ -0,0 +1,10 @@
+use path_editor_core::registry;
+
+#[tauri::command]
+pub fn load_system_paths() -> Result<Vec<String>, String> { registry::load_system_paths() }
+#[tauri::command]
+pub fn load_user_paths() -> Result<Vec<String>, String> { registry::load_user_paths() }
+#[tauri::command]
+pub fn save_system_paths(paths: Vec<String>) -> Result<(), String> { registry::save_system_paths(paths) }
+#[tauri::command]
+pub fn save_user_paths(paths: Vec<String>) -> Result<(), String> { registry::save_user_paths(paths) }