feat: Add a persistent file explorer on wide screens

[satouriko]

Description

The workspace file explorer is currently implemented as a permanent editor tab. On wide screens, this makes it difficult to browse files while keeping code visible at the same time.

Even when the Files tab is moved into its own split pane, selecting a file opens it in that active pane and replaces the visible file tree. The user must switch back to Files after every selection.

Steps to reproduce

1. Open a workspace on a wide screen.
2. Split the editor so the Files tab is visible beside a code tab.
3. Click another file in the file tree.
4. Observe that the Files pane switches to the selected file and the tree is hidden.

Suggested behavior

On wide screens, render the workspace file explorer as a persistent panel outside the editor tab and split layout:

• show it by default beside the editor;
• allow the panel to be resized and collapsed;
• open selected files in the last focused editor group without hiding the tree;
• keep the existing Files tab behavior on narrow screens where a persistent panel would consume too much space.

This would make the workspace behave more like desktop editors while preserving the current mobile interaction.