[FEATURE]: Automatically group desktop session tabs by project

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

In the new OpenCode Desktop layout, all session tabs are displayed in one flat tab bar.

When several projects and sessions are open at the same time, sessions from the same project are mixed with sessions from other projects. Session titles do not reliably indicate their project, making it difficult to identify the correct workspace and increasing the risk of working in the wrong project.

It would be helpful to group session tabs automatically by project/workspace, similar to browser tab groups:

• Show the project name as the group label
• Place new sessions into their project's group automatically
• Allow project groups to be collapsed and expanded
• Visually separate groups using color or clear dividers
• Preserve grouping and tab order after restarting the desktop app

This is related to #37273 and #38525, but specifically requests project-based grouping within the existing horizontal tab layout.