Phase 17: multi-host dashboard (0.19.0)

The dashboard now renders a stacks-with-usage table per host: the local host
plus a section for each registered agent (online dot + offline notice), reusing
the same CPU/memory meters and inline start/stop/restart actions.

- agent_app.py: GET /agent/stacks/stats (reuses stats_service); /agent/system
  now also returns cpu_cores + mem_total for remote meter references.
- routers/agents.py: proxy GET /api/agents/{id}/stacks/stats (declared before
  /{agent_id}/stacks/{stack_id}).
- Frontend: agentsApi.system + stackStats; Dashboard refactored into a shared
  StacksTable used by the local section and a per-agent AgentDashboardSection.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/api/agents.ts

@@ -1,9 +1,19 @@
 import api from "./client";
-import type { Agent, StackDetail, StackSummary } from "@/types";
+import type { Agent, StackDetail, StackStats, StackSummary } from "@/types";
 
 export type RemoteStackSummary = StackSummary & { agent_id: number; agent_name: string };
 export type RemoteStackDetail = StackDetail & { agent_id: number; agent_name: string };
 
+export interface AgentSystem {
+  hostname: string;
+  docker_version: string;
+  host_os: string;
+  cpu_cores: number;
+  mem_total: number;
+  containers_running: number;
+  containers_total: number;
+}
+
 export const agentsApi = {
   list: (refresh = true) =>
     api.get<Agent[]>(`/api/agents?refresh=${refresh}`).then((r) => r.data),
@@ -15,8 +25,12 @@ export const agentsApi = {
   ping: (id: number) =>
     api.post<{ status: string; hostname?: string }>(`/api/agents/${id}/ping`).then((r) => r.data),
 
+  system: (id: number) =>
+    api.get<AgentSystem>(`/api/agents/${id}/system`).then((r) => r.data),
   stacks: (id: number) =>
     api.get<RemoteStackSummary[]>(`/api/agents/${id}/stacks`).then((r) => r.data),
+  stackStats: (id: number) =>
+    api.get<Record<string, StackStats>>(`/api/agents/${id}/stacks/stats`).then((r) => r.data),
   createStack: (id: number, body: { name: string; yaml: string; env?: string }) =>
     api
       .post<{ id: string; name: string }>(`/api/agents/${id}/stacks`, body)