feat: system monitoring + push agent (merge plan M3)

systems + system_stats tables; agents push JSON every N seconds
to POST /api/systems/ingest with a per-system bearer token. The
server stores each sample, keeps a `latest` snapshot for cheap
list/widget reads, prunes stats after 7d, and marks systems down
when silent > max(3x interval, 30s) — firing system.up/down
webhook events on transitions.

cmd/dash-agent is a standalone Linux collector: /proc for
cpu/mem/net/load/uptime, /sys hwmon temps, statfs disk, and
container liveness via /var/run/docker.sock when present.
No imports from the server module — the sample shape is
duplicated deliberately so the agent stays a single static
binary.

Frontend: /systems card grid with usage bars, /systems/:id with
recharts cpu/mem/net/load, temps + containers tables, and the
agent command; `system` board widget; nav + palette + WidgetDialog
picker wired.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/frontend/src/App.tsx

@@ -1,6 +1,6 @@
 import { useEffect, useState } from "react";
 import { Link, Route, useLocation } from "wouter";
-import { Activity, Globe, LayoutGrid, Moon, Plus, Search, Settings, Sun } from "lucide-react";
+import { Activity, Globe, LayoutGrid, Moon, Plus, Search, Server, Settings, Sun } from "lucide-react";
 import { BoardProvider, useBoard } from "@/board/store";
 import { Board } from "@/board/Board";
 import { ServiceDialog } from "@/components/ServiceDialog";
@@ -11,6 +11,8 @@ import { MonitorsPage } from "@/pages/MonitorsPage";
 import { MonitorDetailPage } from "@/pages/MonitorDetailPage";
 import { DomainsPage } from "@/pages/DomainsPage";
 import { DomainDetailPage } from "@/pages/DomainDetailPage";
+import { SystemsPage } from "@/pages/SystemsPage";
+import { SystemDetailPage } from "@/pages/SystemDetailPage";
 import { Button } from "@/components/ui/button";
 import { cn } from "@/lib/utils";
 import type { Item } from "@/types";
@@ -27,6 +29,7 @@ const NAV = [
 	{ href: "/", label: "Board", icon: LayoutGrid },
 	{ href: "/monitors", label: "Monitors", icon: Activity },
 	{ href: "/domains", label: "Domains", icon: Globe },
+	{ href: "/systems", label: "Systems", icon: Server },
 ];
 
 function Shell() {
@@ -141,6 +144,12 @@ function Shell() {
 			<Route path="/domains/:id">
 				{(p) => <DomainDetailPage id={p.id} />}
 			</Route>
+			<Route path="/systems">
+				<SystemsPage />
+			</Route>
+			<Route path="/systems/:id">
+				{(p) => <SystemDetailPage id={p.id} />}
+			</Route>
 
 			<ServiceDialog open={svcOpen} onOpenChange={setSvcOpen} item={editing?.kind === "service" ? editing : undefined} />
 			<WidgetDialog open={wdgOpen} onOpenChange={setWdgOpen} item={editing?.kind === "widget" ? editing : undefined} />