[FEATURE] Desktop: show a "finished, waiting for you" dot on sessions 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

The session list in the desktop app shows a blinking dot while a session is working, but nothing once it finishes. With many sessions running in parallel (I run ~16, mostly Remote Control sessions on another Mac), there's no way to see at a glance which ones are done and waiting for me without clicking through each one.

Proposed Solution

When a session's turn ends, show a persistent filled dot (e.g. blue) next to its name that clears when I open the session, the way ChatGPT/Codex marks finished conversations. It should work for Remote Control sessions too, and be distinct from the yellow "needs input" state.

Alternative Solutions

The existing "task complete" notification banner only fires when the app is in the background, so it doesn't help while you're working in another session in the same window.

Additional Context

Related: #51831 (unread state not clearing on macOS), #59740 ("needs input" state).