feat: system/http/traefik collectors and docker auto-discovery (M3)

Adds the remaining monitor types: a systeminformation-backed system
widget (single-flight cached so multiple widgets don't force
concurrent samples), an http widget with per-widget consecutive-
failure tracking, and a traefik widget that reads router/entrypoint/
middleware status from Traefik's own API. Also adds opt-in label-based
docker auto-discovery (discovery.docker in config.yml): containers
carrying traefik.enable=true are turned into docker widgets in a
synthetic "Discovered" group, re-scanned on a timer and whenever
config.yml changes, with manual widgets always taking precedence over
a discovered one for the same container.

Fixes a real bug surfaced while testing discovery: Next's App Router
bundles app/** through its own compiler pass, separate from server.ts
(run directly via tsx), so configStore/effectiveConfigStore were
silently instantiated twice - one instance watched and updated by
server.ts, another frozen instance read by SSR. Live config edits were
never reflected on page load without a full process restart. Both
stores now key their singleton off globalThis, which both module
graphs share within the same process.

Verified against a real Traefik v3 container (API-driven router list,
ping-based http check) and a labeled nginx container (auto-discovery),
including that a config.yml edit now shows up in a fresh page load
without restarting the server, in both dev and the production build.

lib/collectors/http.ts

@@ -0,0 +1,18 @@
+import type { HttpCheckResult } from "@/lib/types/http-result";
+
+export async function collectHttp(
+  url: string,
+  method: string,
+  timeoutMs: number,
+  expectedStatus: number
+): Promise<Omit<HttpCheckResult, "consecutiveFailures">> {
+  const start = performance.now();
+  try {
+    const response = await fetch(url, { method, signal: AbortSignal.timeout(timeoutMs), redirect: "follow" });
+    const latencyMs = Math.round(performance.now() - start);
+    return { up: response.status === expectedStatus, statusCode: response.status, latencyMs, error: null };
+  } catch (err) {
+    const latencyMs = Math.round(performance.now() - start);
+    return { up: false, statusCode: null, latencyMs, error: (err as Error).message };
+  }
+}