feat(dashboard): nested-card document structure tree

apps/web/app/(dashboard)/dashboard/documents/[docId]/page.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import { useState, useCallback, use } from "react";
+import { useState, useCallback, use, useMemo } from "react";
 import Link from "next/link";
 import { useRouter } from "next/navigation";
 import useSWR from "swr";
@@ -24,12 +24,13 @@ import {
   FileType,
   GitBranch,
   Loader2,
-  ExternalLink,
   Network,
   FileText,
   Download,
   Copy,
   Check,
+  ChevronRight,
+  ChevronDown,
 } from "lucide-react";
 import { MarkdownSummary } from "@/components/ui/markdown";
 import {
@@ -58,6 +59,8 @@ interface DocumentDetail {
 interface TocSection {
   section_id: string;
   order: number;
+  depth?: number;
+  parent_id?: string;
   title: string;
   summary: string;
   page_range: string;
@@ -72,6 +75,148 @@ interface TocData {
   sections: TocSection[];
 }
 
+// ── Nested-card document tree ────────────────────────────────────────
+// Builds a parent→children index from the flat section list (each section
+// carries parent_id + depth) and renders it as collapsible nested cards.
+// Documents with no hierarchy degrade gracefully to a flat list of cards.
+
+function buildSectionTree(sections: TocSection[]) {
+  const byId = new Map(sections.map((s) => [s.section_id, s]));
+  const childrenOf = new Map<string, TocSection[]>();
+  const roots: TocSection[] = [];
+  for (const s of sections) {
+    const pid = s.parent_id ?? "";
+    if (pid && pid !== s.section_id && byId.has(pid)) {
+      const arr = childrenOf.get(pid) ?? [];
+      arr.push(s);
+      childrenOf.set(pid, arr);
+    } else {
+      roots.push(s);
+    }
+  }
+  const byOrder = (a: TocSection, b: TocSection) =>
+    (a.order ?? 0) - (b.order ?? 0);
+  roots.sort(byOrder);
+  childrenOf.forEach((arr) => arr.sort(byOrder));
+  return { childrenOf, roots };
+}
+
+function SectionTree({
+  sections,
+  docId,
+}: {
+  sections: TocSection[];
+  docId: string;
+}) {
+  const { childrenOf, roots } = useMemo(
+    () => buildSectionTree(sections),
+    [sections],
+  );
+  return (
+    <div className="space-y-2">
+      {roots.map((s) => (
+        <SectionTreeNode
+          key={s.section_id}
+          section={s}
+          childrenOf={childrenOf}
+          docId={docId}
+          depth={0}
+        />
+      ))}
+    </div>
+  );
+}
+
+function SectionTreeNode({
+  section,
+  childrenOf,
+  docId,
+  depth,
+}: {
+  section: TocSection;
+  childrenOf: Map<string, TocSection[]>;
+  docId: string;
+  depth: number;
+}) {
+  const kids = childrenOf.get(section.section_id) ?? [];
+  const hasKids = kids.length > 0;
+  // Top two levels open by default; deeper levels start collapsed.
+  const [open, setOpen] = useState(depth < 2);
+
+  return (
+    <div>
+      <div className="rounded-2xl border border-border-light bg-white transition-all hover:border-primary/30 hover:shadow-[rgba(0,0,0,0.06)_0px_2px_10px]">
+        <div className="flex items-start gap-2 p-4">
+          {hasKids ? (
+            <button
+              type="button"
+              onClick={() => setOpen((v) => !v)}
+              aria-label={open ? "Collapse subsections" : "Expand subsections"}
+              className="mt-0.5 shrink-0 rounded-md p-0.5 text-muted-foreground transition-colors hover:bg-black/5 hover:text-foreground"
+            >
+              {open ? (
+                <ChevronDown className="h-4 w-4" />
+              ) : (
+                <ChevronRight className="h-4 w-4" />
+              )}
+            </button>
+          ) : (
+            <span className="mt-0.5 inline-block w-5 shrink-0" />
+          )}
+
+          <span className="mt-0.5 flex h-6 w-6 shrink-0 items-center justify-center rounded-md bg-primary/10 font-data text-xs font-medium text-primary">
+            {section.order ?? "•"}
+          </span>
+
+          <div className="min-w-0 flex-1 space-y-1">
+            <div className="flex flex-wrap items-center gap-2">
+              <Link
+                href={`/dashboard/documents/${docId}/sections/${section.section_id}`}
+                className="text-sm font-medium text-foreground transition-colors hover:text-primary"
+              >
+                {section.title}
+              </Link>
+              {hasKids && (
+                <Badge variant="secondary" className="font-data text-[10px]">
+                  {kids.length} sub
+                </Badge>
+              )}
+            </div>
+            {section.summary && (
+              <div className="text-secondary">
+                <MarkdownSummary content={section.summary} />
+              </div>
+            )}
+          </div>
+
+          {section.page_range && (
+            <Badge
+              variant="secondary"
+              className="shrink-0 font-data text-xs"
+            >
+              pp. {section.page_range}
+            </Badge>
+          )}
+        </div>
+      </div>
+
+      {hasKids && open && (
+        <div className="ml-[1.35rem] mt-2 space-y-2 border-l border-border-gray pl-4">
+          {kids.map((k) => (
+            <SectionTreeNode
+              key={k.section_id}
+              section={k}
+              childrenOf={childrenOf}
+              docId={docId}
+              depth={depth + 1}
+            />
+          ))}
+        </div>
+      )}
+    </div>
+  );
+}
+
 // Engine status enum: pending | parsing | summarizing | ready | failed.
 // Anything that's not ready or failed counts as "in flight" — keep
 // polling until it settles.
@@ -343,48 +488,9 @@ export default function DocumentDetailPage({
                   )}
                 </div>
               )}
-              {sections.map((section, index) => (
-                <Link
-                  key={section.section_id}
-                  href={`/dashboard/documents/${docId}/sections/${section.section_id}`}
-                  className="block rounded-2xl border border-border-light bg-white p-4 transition-all hover:border-primary/30 hover:shadow-[rgba(0,0,0,0.06)_0px_2px_10px]"
-                >
-                  <div className="flex items-start justify-between gap-4">
-                    <div className="flex-1 space-y-1">
-                      <div className="flex items-center gap-2">
-                        <span className="flex h-6 w-6 items-center justify-center rounded-md bg-primary/10 text-xs font-medium text-primary">
-                          {section.order ?? index + 1}
-                        </span>
-                        <h3 className="text-sm font-medium text-foreground">
-                          {section.title}
-                        </h3>
-                      </div>
-                      {section.summary && (
-                        <div className="pl-8">
-                          <MarkdownSummary content={section.summary} />
-                        </div>
-                      )}
-                    </div>
-                    <div className="flex shrink-0 flex-col items-end gap-1">
-                      {section.page_range && (
-                        <Badge
-                          variant="secondary"
-                          className="text-xs font-mono"
-                        >
-                          pp. {section.page_range}
-                        </Badge>
-                      )}
-                      <Badge
-                        variant="outline"
-                        className="text-[10px] font-mono text-muted-foreground"
-                      >
-                        <ExternalLink className="mr-1 h-2.5 w-2.5" />
-                        {section.section_id}
-                      </Badge>
-                    </div>
-                  </div>
-                </Link>
-              ))}
+              {sections.length > 0 && (
+                <SectionTree sections={sections} docId={docId} />
+              )}
             </CardContent>
           </Card>
         </div>