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/graph_api.py

@@ -527,7 +527,7 @@ async def build_node_neighborhood(
       - ``pa:<token>`` — a practice-area hub; same as topic.
     """
     types = normalize_node_types(node_types)
-    depth = max(1, min(int(depth), 2))
+    depth = max(1, min(int(depth), 3))  # BFS is still bounded by NODE_CAP_MAX
     prefix, _, rest = node_id.partition(":")
     rest = rest.strip()
     if prefix not in {"cl", "tag", "pa"} or not rest: