fix(api): normalize null lists to [] (backend serializes empty as null)

The Go backend serializes empty slices as JSON null in several places
(e.g. {count:0, topics:null} when no rows match). The SPA assumed
arrays and crashed on .length:

  TypeError: Cannot read properties of null (reading 'length')
    at Hh (TopicList.tsx:64:45)

Fix in api.ts so every list-returning helper normalizes null → [] once,
centrally. Three call sites covered:
  - listTopics       — topics field
  - listArguments    — arguments field
  - getAgentSummary  — recent_topics field

Components stay as-is; nullable handling moves out of UI code where it
belongs in the data layer. getTopic preserves camps:null vs camps:[]
distinction (null = not yet allocated, [] would be allocated-with-zero
which the allocator contract forbids — keep them meaningfully distinct).

No backend change. Tested on prod with empty topics — list page now
renders 'no topics match this filter' instead of crashing.

src/api.ts

@@ -64,7 +64,12 @@ async function request<T>(
 // --------------------------------------------------------------------
 // Topics
 
-export function listTopics(filter: {
+// Note on null vs []: the Go backend serializes empty slices as JSON
+// null (`json:",omitempty"`-ish behavior in places), so all list-returning
+// endpoints can return e.g. {count:0, topics:null}. Normalize to [] here
+// so consumers can do .length / .map without per-call guards.
+
+export async function listTopics(filter: {
   status?: TopicStatus;
   visibility?: Visibility;
   limit?: number;
@@ -77,23 +82,35 @@ export function listTopics(filter: {
   if (filter.limit) qs.set('limit', String(filter.limit));
   if (filter.offset) qs.set('offset', String(filter.offset));
   const suffix = qs.toString() ? `?${qs}` : '';
-  return request('GET', `/topics${suffix}`, { signal: filter.signal });
+  const r = await request<{ topics: Topic[] | null; count: number }>(
+    'GET',
+    `/topics${suffix}`,
+    { signal: filter.signal },
+  );
+  return { topics: r.topics ?? [], count: r.count ?? 0 };
 }
 
-export function getTopic(
+export async function getTopic(
   id: string,
   signal?: AbortSignal,
 ): Promise<TopicDetail> {
-  return request('GET', `/topics/${encodeURIComponent(id)}`, { signal });
+  const t = await request<TopicDetail>('GET', `/topics/${encodeURIComponent(id)}`, { signal });
+  // camps is nullable pre-allocation; keep null vs [] meaningful — null
+  // means "not yet allocated" (UI shows '—'), [] would mean "allocated
+  // but no rows" (impossible by allocator contract, but distinct).
+  return t;
 }
 
-export function listArguments(
+export async function listArguments(
   topicId: string,
   signal?: AbortSignal,
 ): Promise<{ arguments: Argument[]; count: number }> {
-  return request('GET', `/topics/${encodeURIComponent(topicId)}/arguments`, {
-    signal,
-  });
+  const r = await request<{ arguments: Argument[] | null; count: number }>(
+    'GET',
+    `/topics/${encodeURIComponent(topicId)}/arguments`,
+    { signal },
+  );
+  return { arguments: r.arguments ?? [], count: r.count ?? 0 };
 }
 
 export function getVerdict(
@@ -112,14 +129,16 @@ export function getVerdict(
 // --------------------------------------------------------------------
 // Admin
 
-export function getAgentSummary(
+export async function getAgentSummary(
   agentId: string,
   signal?: AbortSignal,
 ): Promise<AgentSummary> {
-  return request('GET', `/admin/agents/${encodeURIComponent(agentId)}`, {
-    admin: true,
-    signal,
-  });
+  const r = await request<AgentSummary & { recent_topics: AgentSummary['recent_topics'] | null }>(
+    'GET',
+    `/admin/agents/${encodeURIComponent(agentId)}`,
+    { admin: true, signal },
+  );
+  return { ...r, recent_topics: r.recent_topics ?? [] };
 }
 
 export { HttpError };