[Bug]: Closing all terminals clears center panel file browser; must re-open workspace to browse files

### Operating system

_Unknown_

### Orca version

_Unknown_

### Details

**Short summary:** Closing all terminals/agent windows in a workspace clears the center panel file browser even though the workspace is still selected. The UI shows "Select a workspace to browse files", and the user must re-click the workspace and open a terminal before they can browse files or use Git.

### What happened?

The user closed every terminal or agent window in the current workspace. They still considered themselves in that workspace (nothing else was open), but the center panel already showed the empty state **"Select a workspace to browse files"**.

To browse files or use Git again, they had to click the workspace again and open a terminal — which is inconvenient when they only want to browse files (no terminal needed).

### Steps to reproduce

1. Open a workspace in Orca (with terminal and/or agent windows).
2. Ensure the center panel can browse files / show Git for that workspace.
3. Close all terminals and agent windows in that workspace.
4. Observe the center panel empty state: "Select a workspace to browse files".
5. Try to browse files or use Git without opening a new terminal.

### Expected behavior

- Closing terminals/agent windows should not deselect the active workspace for file browsing / Git.
- As long as a workspace remains selected/active, the center panel should keep offering file tree, editor, and Git for that workspace — even with zero open terminals.

### Actual behavior

- After the last terminal/agent window is closed, the center panel switches to "Select a workspace to browse files".
- The user must re-select the workspace and open a terminal before browsing files or using Git.

### Additional context

User report:

> There might be an interaction issue that could be improved. When I close all the terminal or agent windows in the current workspace, theoretically I am still in the current workspace but with nothing open. However, the center panel already displays "Select a workspace to browse files" at that time. I need to click on the workspace again to open a terminal in order to browse files or use Git. This can be inconvenient if all one wants to do is simply browse files.


**Likely area:** workspace selection / panel empty-state logic treating "no open panes/terminals" as "no active workspace", instead of preserving the selected workspace for file browser + Git.