Add SWR for system health and command palette search fetching

Replace manual fetch/useState/useEffect patterns with SWR hooks in
system health section and command palette search, gaining automatic
caching, request deduplication, and stale-while-revalidate.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/swr/fetcher.ts

@@ -0,0 +1,8 @@
+export async function fetcher<T>(url: string): Promise<T> {
+  const res = await fetch(url);
+  if (!res.ok) {
+    const body = await res.json().catch(() => ({}));
+    throw new Error(body.error || `Request failed (${res.status})`);
+  }
+  return res.json();
+}