New SessionDescriptionEditor renders even when empty (with an "Add a
description..." placeholder) and autosaves on typing - same debounced,
no-Save-button pattern as the title and device name editors. Unlike
the name field, an empty description is a valid saved state.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WzyfRyA7h5rs5SCAahLAWd
