Sidebar nav: full garden tree on every page

- buildNavTree: every non-draft page nested by vault folder; folders link
  to their (real or synthetic) index page and take its title when set
- NavTree component: recursive details/summary tree, no JS — folders on
  the current path render open, current page gets aria-current styling
- layout: sticky left sidebar >=72rem, flows after the footer as a site
  index on narrow screens

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/components/NavTree.astro

@@ -0,0 +1,38 @@
+---
+interface Props {
+  nodes: any[];
+  currentPath: string;
+}
+const { nodes, currentPath } = Astro.props;
+// build-time pathnames may carry .html / index.html suffixes — normalize
+const norm = decodeURIComponent(currentPath)
+  .replace(/index\.html$/, '')
+  .replace(/\.html$/, '');
+const here = (u: string) => norm === u || norm === u.replace(/\/$/, '');
+const under = (u: string) => norm.startsWith(u);
+---
+
+<ul class="nav-tree">
+  {
+    nodes.map((n) =>
+      n.children ? (
+        <li>
+          <details open={under(n.url)}>
+            <summary>
+              <a href={n.url} aria-current={here(n.url) ? 'page' : undefined}>
+                {n.name}
+              </a>
+            </summary>
+            <Astro.self nodes={n.children} currentPath={currentPath} />
+          </details>
+        </li>
+      ) : (
+        <li>
+          <a href={n.url} aria-current={here(n.url) ? 'page' : undefined}>
+            {n.name}
+          </a>
+        </li>
+      ),
+    )
+  }
+</ul>