Add search functionality to Independent Panel chat history, close #550

Add a sidebar search with debounced, diacritic-insensitive filtering
across conversation titles and content. Include i18n strings for the
search UI.

Keep the panel responsive and accessible: precompute a normalized
per-session index to keep typing smooth on long lists, improve keyboard
navigation and shortcuts to focus the search and temporarily expand the
sidebar, and move sidebar styling into SCSS.

Persist and reliably sync the sidebar's collapsed state across reloads,
handling storage failures without blocking the UI.

Harden deletion and clear flows, sanitize inputs, and guard async
updates to keep state consistent.

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: qodo-merge-pro[bot] <151058649+qodo-merge-pro[bot]@users.noreply.github.com>