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>

assets/tree.html

@@ -0,0 +1,18 @@
+{{/* Folder tree rows (tree.go), shared by the sidebar and the tree picker.
+Expandable folders are native <details>: open/close needs no script, and a
+folder's children load via htmx the first time it opens, unless the server
+pre-expanded them. A label inside <summary> selects its radio without
+toggling the folder; the rest of the row toggles. */}}
+
+{{define "rows"}}{{range .}}{{if .Expandable}}<details class="tree-node"{{if .Children}} open{{end}}>
+<summary class="{{template "row-class" .}}"><span class="tree-chevron" aria-hidden="true"></span>{{template "label" .}}</summary>
+<div class="tree-children"{{if not .Children}} hx-get="{{.ChildrenURL}}" hx-trigger="toggle once from:closest details"{{end}}>{{template "rows" .Children}}</div>
+</details>
+{{else}}<div class="{{template "row-class" .}}"><span class="tree-chevron is-leaf" aria-hidden="true"></span>{{template "label" .}}</div>
+{{end}}{{end}}{{end}}
+
+{{define "row-class"}}tree-row{{if .Active}} is-active{{end}}{{if and (not .Link) (not .Selectable)}} is-disabled{{end}}{{end}}
+
+{{define "label"}}{{if .Link}}<a class="tree-name tree-{{.Kind}}" href="{{.URL}}">{{.Name}}</a>
+{{- else if .Selectable}}<label class="tree-name"><input type="radio" name="pick" value="{{.Path}}" data-kind="{{.Kind}}"{{if .Checked}} checked{{end}} />{{.Name}}</label>
+{{- else}}<span class="tree-name">{{.Name}}</span>{{end}}{{end}}