Show an explicit empty state for projects with no threads in the sidebar

[CyMule]

Right now, when a project has no chat threads, expanding it in the sidebar only changes the chevron and shows a blank area. Adding a minimal No threads yet label makes the empty state immediately clear.

Prompt:

When a project in the left sidebar has zero chat threads, show a small inline empty state instead of leaving the expanded area blank.
Keep it minimal and unobtrusive, for example a muted No threads yet label inside the project section.
The goal is to make it immediately clear that the project is empty when the user expands it.

Current behavior:

sidebar-empty-state-before-demo.webm

Suggested behavior:

sidebar-empty-state-demo.webm