feat(client): add unread indicator for completed sessions

[Master-cai]

Summary

This PR adds an unread indicator for sessions that complete work while not currently selected.

Changes

• Add unreadSessionIds state to session store.
• Add store actions:
  • markSessionUnread(sessionId)
  • markSessionRead(sessionId)
• Mark a session unread when status transitions from working -> waiting and the session is not selected.
• Clear unread state when the user selects that session.
• Clean up stale unread session ids when sessions are removed.
• Persist unread ids in store persistence (serialize/deserialize Set correctly).
• Add UI unread dot indicator in session row.
• Add unread indicator styles (with reduced-motion fallback).
• Add tests for unread mark/read/cleanup behavior in sessionState tests.

Why

When multiple agents run concurrently, it’s easy to miss that a background session has completed.
The unread indicator makes finished work discoverable without forcing a session switch.

Scope

• Client-only change (SessionList, client store, styles, tests)
• No server API contract changes

Verification

• bun run lint passed
• bun run typecheck passed
• bun run test still has existing unrelated baseline failures in this repository (server-side test isolation/logging cleanup issues), not introduced
  by this PR.