[FEATURE] Desktop sidebar: color-coded session status (done / waiting-on-me / needs-attention), not just busy/idle

Problem

In the Desktop app sidebar, sessions show a small circle indicator, but it only distinguishes busy vs idle vs asleep. There's no way to tell at a glance which idle sessions are actually finished with nothing further needed vs waiting on me to answer/decide something vs stalled on an error. With 30-40 sessions in a project list, they all read as the same gray, and a user has to open each one to find out which need attention.

Requested behavior

Color-code the session indicator (or add a second small dot) by a coarse status, independent of the busy/idle animation:

• 🟢 green — turn ended cleanly, nothing pending
• 🟡 yellow — Claude is waiting on user input/approval/decision
• 🔴 red — turn ended on an error or a failing check
• 🔵 blue — session's task is fully complete (e.g. PR merged) and can be archived

Today users have to fake this with an emoji prefix in the session title (which works, but eats the title's real estate and isn't automatic).

Why now

Yellow-for-waiting-on-input already exists in terminal agent-view per the docs (https://code.claude.com/docs/en/agent-view.md) — this is asking for that same signal, plus a "done, nothing pending" and "complete/archivable" state, to appear in the Desktop app's sidebar circle itself rather than only in agent-view.

Related but distinct: #87508 asks for a working-vs-stopped indicator in the VS Code extension. This request is about the Desktop app's idle-state sessions needing a completion/attention color, not the busy/idle distinction.