fix(editor): asset browser directory navigation

The file list was hardwired to the root directory, so it was always
empty (all assets live in subdirectories). Directories are now
selectable from both the tree (click row) and the list (click [D]
row), with a breadcrumb + Root button. Prefix check done manually
(path::starts_with is missing in this libstdc++).

App/editor/AssetBrowserPanel.h

@@ -31,6 +31,8 @@ private:
 
     // 固定根（下标即身份）：项目资产 / 场景文件
     int m_Root = 0;
+    // 当前浏览目录（点目录树切换；列表显示这里的文件）
+    std::filesystem::path m_CurrentDir;
     // 类型过滤（0 = All；其余见 kTypeNames）
     int m_TypeFilter = 0;
     // 名字子串过滤（空 = 不过滤）