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/lib/nav.mjs

@@ -0,0 +1,55 @@
+// Sidebar navigation tree: every non-draft page in the garden, nested by
+// vault folder. Folders link to their index page (real or synthetic).
+import { urlForId, slugForId } from './quartz-compat.mjs';
+
+export function buildNavTree(entries) {
+  const root = { children: new Map() };
+
+  const dirNode = (parts) => {
+    let node = root;
+    let acc = '';
+    for (const seg of parts) {
+      acc = acc ? `${acc}/${seg}` : seg;
+      if (!node.children.has(seg)) {
+        node.children.set(seg, {
+          name: seg,
+          url: '/' + slugForId(acc) + '/',
+          children: new Map(),
+        });
+      }
+      node = node.children.get(seg);
+    }
+    return node;
+  };
+
+  for (const e of entries) {
+    if (e.data.draft || e.id === 'index') continue;
+    const parts = e.id.split('/');
+    const base = parts.pop();
+    const folder = dirNode(parts);
+    if (base === 'index') {
+      // folder's own page: use its title for the folder label if set
+      if (e.data.title) folder.name = e.data.title;
+      continue;
+    }
+    folder.children.set(base, {
+      name: e.data.title ?? base,
+      url: urlForId(e.id),
+      children: null,
+    });
+  }
+
+  const finalize = (node) => {
+    const kids = [...node.children.values()].map((k) =>
+      k.children ? finalize(k) : k,
+    );
+    kids.sort((a, b) => {
+      const af = a.children ? 0 : 1;
+      const bf = b.children ? 0 : 1;
+      return af - bf || a.name.localeCompare(b.name, 'en', { sensitivity: 'base' });
+    });
+    return { ...node, children: kids };
+  };
+
+  return finalize(root).children;
+}