feat: live docker/database widgets over WebSocket (M2)

Adds a custom server.ts (http server + Next request handler + a
noServer:true WebSocket server on /ws) so the app can push live data
without polling. A CollectorScheduler runs one interval-based job per
docker/database widget instance, backed by dockerode against
/var/run/docker.sock, and reconciles jobs when config.yml changes.

Config hot-reload is now fully wired: chokidar watches config.yml/.env,
re-validates on change, and broadcasts config:update (or a
non-destructive config:error, keeping the last-good config) to every
connected browser. The client subscribes to per-widget topics over a
single shared WebSocket with exponential-backoff reconnect and
last-result caching for instant resubscribe.

Verified end-to-end against real throwaway containers (nginx, redis):
live CPU/mem/uptime/health streamed over the socket, interval changes
picked up without a server restart, and a broken config.yml correctly
falls back to the last-valid config instead of crashing the app.

components/layout/Dashboard.tsx

@@ -1,14 +1,19 @@
+"use client";
+
 import type { Config } from "@/lib/config/schema";
+import { useConfigSubscription } from "@/lib/ws/client";
 import { GroupSection } from "./GroupSection";
 
-export function Dashboard({ config }: { config: Config }) {
+export function Dashboard({ config: initialConfig }: { config: Config }) {
+  const config = useConfigSubscription(initialConfig);
+
   return (
     <main className="mx-auto flex max-w-6xl flex-col gap-8 px-6 py-10">
       <header className="flex items-center justify-between">
         <h1 className="text-lg font-semibold text-fg">{config.settings.title}</h1>
       </header>
-      {config.groups.map((group) => (
-        <GroupSection key={group.name} group={group} />
+      {config.groups.map((group, groupIndex) => (
+        <GroupSection key={group.name} group={group} groupIndex={groupIndex} />
       ))}
     </main>
   );