Folder sections now collapse via an explicit chevron/tap header
(rather than relying on platform-dependent native disclosure, which
didn't render/click reliably) — same collapsed-state binding also
backs Section(isExpanded:) so content visibility stays in sync.
Conversations are draggable onto folder/Unfiled headers to file/unfile
them. Folder names render bold. "New Folder" is now a dedicated button
next to "New Chat" in the sidebar, and next to "Select" in the
advanced conversation list, instead of a small icon buried in the
search row.