Sidebar: optional collapsible project groups

[dpetreleyai]

Problem

All project buttons live in one flat list. Power users organize work in clusters (departments, clients, ICM clusters) and can easily accumulate 50–150 project folders; a flat sidebar cannot hold that.

Intended fix

An optional group field on a project (projects.json, default empty — fully backwards compatible):

• Projects with the same group render under one collapsible header (name + count). Named groups start collapsed; the expanded set persists in the UI settings. Ungrouped projects render first, ungrouped, exactly as today.
• While a filter query is active (see the search-input proposal) all matches show regardless of collapse state.
• Editor: a group text input per row with a datalist of existing group names.
• Rust: one #[serde(default)] group: String on the Project struct — pure passthrough.

A fresh install or an existing projects.json without group fields looks and behaves exactly as before.

[astetic-dev]

Acknowledged - deliberately not part of 0.4.0. Still under consideration for a future release; keeping it open on the backlog.