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/agent_app.py

@@ -43,6 +43,7 @@ from services import (
     file_service,
     image_service,
     network_service,
+    stats_service,
     update_service,
     volume_service,
 )
@@ -60,7 +61,7 @@ def _map_docker(exc: DockerError):
         raise HTTPException(status_code=code, detail=exc.detail or exc.error)
     raise exc  # falls through to the global 502 DockerError handler
 
-AGENT_VERSION = "0.18.0"
+AGENT_VERSION = "0.19.0"
 
 
 # --------------------------------------------------------------------------- #
@@ -158,6 +159,18 @@ def _hostname() -> str:
     return os.uname().nodename
 
 
+def _mem_total() -> int:
+    for base in (settings.HOST_PROC_PATH, "/proc"):
+        try:
+            with open(os.path.join(base, "meminfo"), "r", encoding="utf-8") as fh:
+                for line in fh:
+                    if line.startswith("MemTotal:"):
+                        return int(line.split()[1]) * 1024  # kB -> bytes
+        except OSError:
+            continue
+    return 0
+
+
 def _system_info() -> dict:
     docker_version = ""
     host_os = ""
@@ -175,6 +188,8 @@ def _system_info() -> dict:
         "hostname": _hostname(),
         "docker_version": docker_version,
         "host_os": host_os,
+        "cpu_cores": os.cpu_count() or 0,
+        "mem_total": _mem_total(),
         "containers_running": running,
         "containers_total": total,
     }
@@ -207,6 +222,11 @@ def list_stacks() -> list[dict]:
     return [_summary(sid) for sid in compose_service.discover_stacks()]
 
 
+@app.get("/agent/stacks/stats", dependencies=[Depends(verify_token)])
+def stacks_stats() -> dict:
+    return stats_service.stack_stats()
+
+
 @app.get("/agent/stacks/{stack_id}", dependencies=[Depends(verify_token)])
 def get_stack(stack_id: str) -> dict:
     directory = compose_service.stack_dir(stack_id)