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>

tree_test.go

@@ -0,0 +1,40 @@
+package main
+
+import (
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestHandleTreeExpandTo(t *testing.T) {
+	root := t.TempDir()
+	for _, d := range []string{"Projects/A/Deep", "Other/B"} {
+		if err := os.MkdirAll(filepath.Join(root, filepath.FromSlash(d)), 0755); err != nil {
+			t.Fatal(err)
+		}
+	}
+	h := &handler{root: root}
+	get := func(urlPath, query string) string {
+		r := httptest.NewRequest("GET", urlPath+"?"+query, nil)
+		w := httptest.NewRecorder()
+		h.handleTree(w, r, urlPath, filepath.Join(root, filepath.FromSlash(strings.Trim(urlPath, "/"))))
+		return w.Body.String()
+	}
+
+	// A chain below the listed folder is pre-expanded.
+	if body := get("/Projects/", "tree&expandTo=/Projects/A"); !strings.Contains(body, `href="/Projects/A/Deep/"`) {
+		t.Errorf("expandTo below the folder did not expand:\n%s", body)
+	}
+	// Lazy children keep the picker options.
+	if body := get("/", "tree&pick=folder&root=1"); !strings.Contains(body, `hx-get="/Projects/?tree&pick=folder"`) ||
+		!strings.Contains(body, `value="/"`) {
+		t.Errorf("picker rows missing lazy URL or root row:\n%s", body)
+	}
+	w := httptest.NewRecorder()
+	h.handleTree(w, httptest.NewRequest("GET", "/?tree&pick=bogus", nil), "/", root)
+	if w.Code != 400 {
+		t.Errorf("bad pick mode: got %d, want 400", w.Code)
+	}
+}