feat(workspace): add no-folder empty states and gate folder-only actions

- Sidebar: replace the "no conversations" placeholder with Open Folder, Clone Repository, and Project Boot buttons when the workspace has no open folders.
- Title bar: disable the terminal and auxiliary-panel toggle buttons while no folder is active.
- Aux panel: show a shared localized "no folder open" prompt in the file tree, git changes, and git log tabs when no folder is active.
- Add auxPanel.noFolderTitle / noFolderHint translations across all ten supported locales.

src/i18n/messages/zh-CN.json

@@ -935,7 +935,9 @@
         "files": "文件",
         "changes": "变更",
         "commits": "提交"
-      }
+      },
+      "noFolderTitle": "暂无打开的文件夹",
+      "noFolderHint": "打开一个文件夹后在这里查看"
     },
     "windowControls": {
       "minimizeWindow": "最小化窗口",