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/GroupSection.tsx

@@ -1,7 +1,8 @@
 import type { Group } from "@/lib/config/schema";
+import { widgetId } from "@/lib/config/widgets";
 import { widgetRegistry } from "@/components/widgets/registry";
 
-export function GroupSection({ group }: { group: Group }) {
+export function GroupSection({ group, groupIndex }: { group: Group; groupIndex: number }) {
   return (
     <section className="flex flex-col gap-3">
       <h2 className="text-xs font-semibold tracking-wide text-fg-muted uppercase">
@@ -10,7 +11,8 @@ export function GroupSection({ group }: { group: Group }) {
       <div className="grid grid-cols-[repeat(auto-fill,minmax(220px,1fr))] gap-3">
         {group.widgets.map((widget, index) => {
           const Component = widgetRegistry[widget.type];
-          return <Component key={index} widget={widget as never} />;
+          const id = widgetId(groupIndex, index);
+          return <Component key={id} widget={widget as never} widgetId={id} />;
         })}
       </div>
     </section>