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>

backend/routers/agents.py

@@ -213,6 +213,16 @@ async def agent_stacks(
     return stacks
 
 
+@router.get("/{agent_id}/stacks/stats")
+async def agent_stacks_stats(
+    agent_id: int,
+    session: Session = Depends(get_session),
+    _user: User = Depends(get_current_user),
+) -> dict:
+    agent = _get_or_404(session, agent_id)
+    return await _proxy(session, agent, "GET", "/agent/stacks/stats")
+
+
 @router.get("/{agent_id}/stacks/{stack_id}")
 async def agent_stack_detail(
     agent_id: int,