Render the folder tree on the server

The sidebar tree is part of the page (chain to the current page
expanded, active row marked), so it no longer paints late and the
sessionStorage snapshot is gone; only the rail's scroll position is
kept. Folders are native <details> that lazy-load their children as
HTML via htmx. The tree picker hosts the same server-rendered rows with
radio inputs, so ?tree returns HTML and the JSON listing is removed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

render.go

@@ -61,6 +61,7 @@ type pageData struct {
 	SpecialContent template.HTML
 	SidebarWidget  template.HTML
 	SuppressTOC    bool
+	Tree           template.HTML // sidebar folder tree; empty in edit mode
 	renderTimer
 }