[FEATURE]: Add named project groups

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

When many projects are open, the project list becomes difficult to scan and unrelated repositories are mixed together. I would like one-level, user-defined project groups that can be named, collapsed, and reordered locally.

Projects could be assigned from their context menu or by dragging them onto a group, and moved back to an Ungrouped section. Group membership would be per-server local UI state and would not change project paths, IDs, or session data.

The closest older request was #15469, and the related sub-project implementation in #16934 closed after inactivity. This proposal differs by using arbitrary named groups rather than parent/child project relationships.

I have a working local prototype for the legacy project sidebar. Before preparing a PR, should this target the legacy layout, the new project UI, or shared state supporting both?