ui : replace the sidebar search mode with a search dialog
Search opens a spotlight dialog (Cmd+K or the Search entry under New chat in the sidebar) with keyboard navigation over recent chats and sidebar actions. Phones keep the search page.
Sidebar actions show their shortcuts, with new ones for Settings (Cmd+,) and Models (Shift+Cmd+M).
Assisted-by: Claude (Opus 5.5 Medium)
