Tolerate null agent in /node response

Public seeds return `"agent": null` because their deployed httpd
binaries can't parse i2p/onion rows that newer nodes write to the
address table — `AddressStore::get` fails and the handler falls back
to `None`. Allow null in the zod schema and hide the user-agent row
in the UI when it's missing.

Workaround until heartwood ships `StoreExt::agent_of` (reads only
the `agent` column, no address hydration) and httpd adopts it.

http-client/index.ts

@@ -97,7 +97,7 @@ export type Node = z.infer<typeof nodeSchema>;
 
 const nodeSchema = z.object({
   id: z.string(),
-  agent: z.string(),
+  agent: z.string().nullable(),
   config: nodeConfigSchema.nullable(),
   state: z.union([z.literal("running"), z.literal("stopped")]),
   avatarUrl: z.string().optional(),