feat(graph): navigation & UX — deep-link, depth, PNG, rich panel (PR D)

Final corpus-graph PR. Connects the graph to the chair's workflow and rounds
out the Obsidian-grade interactions.

Backend (web/graph_api.py): neighborhood depth cap 2 → 3 (still bounded by
NODE_CAP_MAX).

Frontend:
- URL deep-link: /graph?focus=cl:<id> is read on mount and written on focus
  change (router.replace, scroll:false). GraphView wrapped in <Suspense> per
  Next 16's useSearchParams requirement.
- "הצג בגרף" button on the precedent detail page → /graph?focus=cl:<id>.
- Depth slider (1–3) in the focused overlay → useNodeNeighborhood(id, depth).
- Export PNG: grabs the rendered <canvas> from the area ref → toDataURL →
  download; failures surface a toast (UI4).
- Rich node panel: precedent nodes fetch headnote/summary via the existing
  usePrecedent hook (Skeleton while pending, error surfaced — UI4).
- Edge-type legend (ציטוט / נושא-תחום / יומון) added under the node legend.

Deferred (noted for a later pass): expand-in-place merge, search→camera-center.

web-ui build + lint pass. Invariants: G2 (depth change is read-only), UI4
(PNG + detail errors surfaced, not swallowed). api:types post-deploy.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web-ui/src/components/graph/graph-node-panel.tsx

@@ -12,7 +12,9 @@ import { ExternalLink, X } from "lucide-react";
 import { Badge } from "@/components/ui/badge";
 import { Button } from "@/components/ui/button";
 import { Card, CardContent } from "@/components/ui/card";
+import { Skeleton } from "@/components/ui/skeleton";
 import type { GraphNode } from "@/lib/api/graph";
+import { usePrecedent } from "@/lib/api/precedent-library";
 
 const TYPE_LABELS: Record<string, string> = {
   precedent: "פסיקה",
@@ -54,6 +56,9 @@ export function GraphNodePanel({
   const isPrecedentLike = node.type === "precedent" || node.type === "halacha";
   const isGap = node.type === "gap";
   const isDigest = node.type === "digest";
+  // Rich detail (summary/headnote) for precedents — reuses the library hook.
+  const detailId = node.type === "precedent" ? node.case_law_id : null;
+  const detail = usePrecedent(detailId);
   return (
     <Card className="bg-surface border-rule shadow-sm w-80 shrink-0 overflow-y-auto">
       <CardContent className="space-y-4 p-4">
@@ -119,6 +124,25 @@ export function GraphNodePanel({
           )}
         </dl>
 
+        {detailId && (
+          <div className="space-y-1.5 border-t border-rule pt-3">
+            {detail.isPending ? (
+              <>
+                <Skeleton className="h-3 w-full" />
+                <Skeleton className="h-3 w-4/5" />
+              </>
+            ) : detail.error ? (
+              <p className="text-ink-muted text-xs m-0">לא ניתן לטעון תקציר.</p>
+            ) : detail.data?.headnote || detail.data?.summary ? (
+              <p className="text-ink text-sm leading-relaxed m-0">
+                {detail.data.headnote || detail.data.summary}
+              </p>
+            ) : (
+              <p className="text-ink-muted text-xs m-0">אין תקציר.</p>
+            )}
+          </div>
+        )}
+
         {isPrecedentLike && node.case_law_id && (
           <Button asChild variant="outline" className="w-full">
             <Link href={`/precedents/${node.case_law_id}`}>