feat(web): FE overhaul — SWR centralized data layer + dashboard + islands

- Add SWR foundation (lib/fetcher, lib/swr, lib/api) with typed
  hooks: useSections/useDocuments/useSearch/useRelated/useRevisions/useQueueStatus
- Replace ad-hoc useEffect+fetch in Header/Sidebar/CommandMenu/search
  with SWR (dedup 4s, focus throttle 10s, debounced search 180ms)
- Doc page: RelatedGrid + RevisionList as SWR client islands (RSC hero stays)
- DocForm: SWR mutate on create/update for live sidebar invalidation
- New APIs: /api/related, /api/revisions, /api/tags, /api/queue/status,
  /api/docs filtering (?section=&status=)
- New UI: DocumentCard, SectionCard, SearchBar, TagCloud, QueueBadge,
  EmptyState, dashboard (/dashboard with QueueStrip live + section stats)
- Layout wraps SWRProvider; no TanStack code (SWR-only per spec)
- Verified: next build passes (16.3.4 Turbopack), no manual fetch sprawl remains

Co-Authored-By: internal-model

apps/web/app/components/EmptyState.tsx

@@ -0,0 +1,53 @@
+import { FileQuestion, SearchX, AlertCircle, Inbox } from "lucide-react";
+import Link from "next/link";
+
+export function EmptyState({
+  title = "No results",
+  description,
+  icon: Icon = Inbox,
+  action,
+}: {
+  title?: string;
+  description?: string;
+  icon?: typeof Inbox;
+  action?: { label: string; href: string };
+}) {
+  return (
+    <div className="text-center py-12 bg-[var(--bg-surface)] border border-[var(--border-color)] rounded-xl shadow-xs space-y-3">
+      <div className="w-10 h-10 rounded-xl bg-[var(--bg-elevated)] border border-[var(--border-color)] flex items-center justify-center mx-auto text-[var(--text-muted)]">
+        <Icon className="w-5 h-5" />
+      </div>
+      <h3 className="text-sm font-bold text-[var(--text-primary)]">{title}</h3>
+      {description && <p className="text-xs text-[var(--text-muted)] max-w-sm mx-auto leading-relaxed">{description}</p>}
+      {action && (
+        <Link
+          href={action.href}
+          className="inline-flex items-center gap-1.5 text-xs bg-[var(--brand)] text-[var(--brand-fg)] px-3 py-1.5 rounded-md font-semibold mt-2"
+        >
+          {action.label}
+        </Link>
+      )}
+    </div>
+  );
+}
+
+export function ErrorState({ message, onRetry }: { message?: string; onRetry?: () => void }) {
+  return (
+    <div className="p-4 rounded-xl border border-amber-500/30 bg-amber-500/10 text-amber-900 dark:text-amber-200 text-xs flex items-start gap-2.5">
+      <AlertCircle className="w-4 h-4 shrink-0 mt-0.5" />
+      <div className="flex-1 min-w-0">
+        <div className="font-semibold">Something went wrong</div>
+        {message && <div className="text-[11px] opacity-80 mt-1 break-words">{message}</div>}
+      </div>
+      {onRetry && (
+        <button
+          onClick={onRetry}
+          type="button"
+          className="shrink-0 px-2.5 py-1 rounded bg-[var(--bg-surface)] border border-[var(--border-color)] text-[var(--text-primary)] font-semibold hover:bg-[var(--bg-elevated)]"
+        >
+          Retry
+        </button>
+      )}
+    </div>
+  );
+}