Sidebar hierarchy prototype

Project rows use the same first-level layout as Functions/Chats (32px height, font and hover), with 📁 in the icon slot and a trailing chevron; Session rows are second-level: text starts at x=36, height is 28px, font is 13px, and the vertical guide runs through the icon slot center at x=16.

Sessions without a project remain under a subtle Recents section heading, with an 8px left edge; they are not represented as folders.
Expansion persists in localStorage; the project containing the current session opens automatically. Hovering a project row reveals + to create a session in that project.