[FEATURE] Desktop app: user-defined session groups and colors in the sidebar

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

In the Claude Code desktop app, the sessions sidebar offers only the built-in groups (e.g. In Progress, Later, Ungrouped) and a small fixed set of status-dot colors. Once you accumulate dozens of sessions across several workstreams (different repos, PRs, research threads, recurring routines), the flat "Ungrouped" list becomes a long undifferentiated scroll and the few available groups/colors aren't enough to organize it.

Proposed Solution

Let users organize the sidebar themselves:

• Custom groups: create, rename, reorder, and collapse arbitrary user-defined groups (e.g. "PR #1896", "Sprint planning", "Research"), and drag sessions between them — beyond the current fixed In Progress / Later split.
• More colors / custom colors: a larger palette (or a free color picker) for the session indicator dots, assignable per session or per group, so color can encode meaning (project, urgency, workstream).
• Nice-to-have: assign a default group/color per project directory, so new sessions land sorted automatically.

Alternative Solutions

Currently I approximate this by renaming sessions with prefixes (e.g. #1234: …) so related sessions at least sort/scan together by title. That helps but doesn't scale — there's no visual separation and no way to collapse a finished workstream.

Priority

Medium - Would be very helpful

Feature Category

Interactive mode (TUI)

Use Case Example

1. I'm iterating on a single PR across multiple sessions (implementation, testing, CI failures) while also running unrelated planning/research sessions.
2. I create a group "PR #1896", drag the three related sessions into it, and give it a distinct color.
3. When the PR merges, I collapse (or archive) the whole group in one action.
4. The sidebar stays scannable even with 30+ sessions.

Additional Context

Related but distinct: configurable auto-titling of sessions is tracked in #61179 — this request is about manual visual organization (groups + colors) in the desktop sidebar, which complements better titles.