feat(frontend): Topologies page — catalog browser + builder

New authed workspace page (/topologies, in the global nav) that browses the
topology catalog and builds + visualizes a topology from a kind + roles:
- lib/api/topology.ts: zod schemas + fetchTopologyCatalog (server, authed).
- TopologyGraphView: lightweight SVG renderer (no new deps), laid out per kind
  (row for pipeline/ring, star for delegation kinds, circle otherwise).
- TopologyExplorer (client): catalog list + roles input → POST
  /api/topologies/build via the /api proxy → render the graph.
- ShellNav: add the Topologies nav item.
- e2e (p8): sign in → nav → browse catalog → build a pipeline → graph renders.

Full suite green (38); npm build + TS clean. Goes live with the next server+
frontend deploy (compare/Pareto UI to follow).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

frontend/src/app/(workspace)/topologies/page.tsx

@@ -0,0 +1,16 @@
+import { PageChrome } from "@/components/global/PageChrome";
+import { TopologyExplorer } from "@/components/topology/TopologyExplorer";
+import { fetchTopologyCatalog } from "@/lib/api/topology";
+
+// Topologies page: browse the catalog and build/visualize a topology.
+export default async function TopologiesPage() {
+  const catalog = await fetchTopologyCatalog();
+  return (
+    <PageChrome
+      title="Topologies"
+      description={`${catalog.length} organizational patterns to run your agents in`}
+    >
+      <TopologyExplorer catalog={catalog} />
+    </PageChrome>
+  );
+}